“Featured images” are supported in a relatively small number of themes (see below). They are set as shown in the relevant Support doc.
The function of a featured image is theme-dependent. Unfortunately the Support doc mentions emphasizes one of the possible functions only, and has misled many users. [See Update at end of post.] The actual effect is as follows:
Adventure Journal, Chaos Theory, Chateau, Coraline, Duster, Ocean Mist, Pilcrow, Splendio, Twenty Eleven, Twenty Ten, Vostok
Replaces regular header image when viewing single post or static page. For dimensions see my post Header images (cropped if larger, no show if smaller).
Autofocus
Shows up above post title. Main page: top left part shows, rest cropped to fit. Single post view: maximum width 800px (downsized if wider). Category/archive pages: maximum width 595px (downsized if wider), maximum height 211px (cropped if it doesn’t fit).
Dusk to Dawn
Shows up below post title on main page, archive pages, single post view, also on static pages. Maximum width 588px (downsized if larger).
Esquire
Thumbnail replacing post title on Audio format posts (designed to simulate an LP sleeve, with the record halfway out). Dimensions 207×207 px (cropped if larger, no show if smaller).
Imbalance 2
Thumbnail on main page and category/archive pages. Also in footer sticky posts (optional) when viewing single post. Maximum width 210px (downsized if wider).
Matala
Thumbnail below date (on posts only). Width 55 px (downsized when larger).
Modularity Lite
Thumbnail at beginning of post on category/archive pages. Dimensions 150×150 px (cropped if larger, no show if smaller).
Nishita
Thumbnail on category/archive pages. Width 128px (downsized if larger).
Reddle
Shows up above post title on main page, category/archive pages, single post view, also on static pages. Maximum width 441px if you have added a header image (downsized if wider, in black background if narrower).
Selecta
Thumbnail on main page, archive pages, latest posts showcase row. Dimensions 190×110 px (cropped if larger, no show if smaller).
Skeptical
Full-width image at beginning of post (on main page, archive pages, single post view). Width 519px (downsized if larger).
Sunspot
Shows up on main posts page if the two main column layout has been selected. Dimensions 259×159 px (cropped if larger).
Bueno
a) Shows up at beginning of post on main posts page and category/archive pages. Maximum dimensions 490×200 px (cropped if larger).
b) Thumbnail in sidebar if you have activated the “Bueno Featured Posts” widget. Width 70 px (downsized when larger).
See my post Featured posts/images in Bueno.
Forever
a) Shows up in featured posts slider. Width 888px, maximum height 355px (no show if narrower, cropped if wider/higher).
b) Thumbnail on 4-column showcase for recent posts. Width 195px, maximum height 124px (cropped if larger).
c) Shows up in image format posts, on main and category/archive pages. Maximum width 510px (downsized if wider).
Fresh & Clean
a) Shows up in featured posts slider. Width 840px, maximum height 280px (no show if narrower, cropped if wider/higher).
b) Thumbnail in regular posts, on main and category/archive pages. Dimensions 160×120 pixels (downsized and cropped to fit).
iTheme2
a) Shows up above post title on main page, archive pages, single post view, also on static pages. Maximum dimensions 593×261 px (cropped if either dimension is larger).
b) If set to a sticky post, thumbnail in the “carousel” showcase slider of the theme. Maximum dimensions 145×120 px (cropped if either dimension is larger).
Nuntius
a) Thumbnail on category/archive pages. Dimensions 100×75 pixels (downsized and cropped to fit, if larger).
b) If set to a sticky post, shows up in the slider of the “news” template page. Dimensions 640×425 pixels (downsized if wider, blown up if narrower, cropped if higher).
Inuit Types
a) Thumbnail at beginning of post on main posts page.
b) Thumbnail for featured posts (optional) on main posts page
c) Full-width image when viewing single post (optional).
See my post Featured posts/images in Inuit Types for details.
Mystique
a) Thumbnail to the left of the post title on homepage and index pages. Dimensions 90×70 px (downsized when larger, no show if smaller).
b) Thumbnail below title on static pages. Width 55px (downsized when larger).
c) Full-width background to “featured post” on homepage, replacing post content, date and author. Dimensions 940×240 px (cropped if larger, no show if smaller).
Shaan
a) Thumbnail at beginning of post on main page and category/archive pages. Dimensions as set in Settings > Media > Thumbnail.
b) Above title on sticky posts and static pages. Dimensions 600×250 px (cropped if larger, no show if smaller).
Splendio
a) Replaces regular header image when viewing single post or static page. Dimensions 980×174 px (cropped if larger, no show if smaller).
b) Thumbnail on showcase template featured slider. Maximum width 180px (downsized if larger, distorted if smaller).
c) Shows up below title on showcase template latest post. Maximum width 380px (downsized if larger).
d) Shows up below title on main and category/archive pages. Maximum width 600px (downsized if larger).
Structure
a) Large-size image in “Homepage Top” area. Dimensions 620×380 px (cropped if larger and different aspect ratio, no show if smaller).
b) Full-width image when viewing single post (optional). Dimensions 640×392 px (cropped if larger, no show if smaller).
c) Full-width image at beginning of post on main posts page and category/archive pages. Dimensions 440×240 px.
Note that 620×380 and 640×392 are the same aspect ratio but 440×240 isn’t: if you upload images properly sized for the Homepage Top area and/or the single post view, the top and bottom of version c will be slightly cropped.
See my post Introduction to the Structure theme as well.
Suburbia
a) Thumbnail on main and category/archive pages. Dimensions 155×110 px (distorted if different).
b) Medium sized image on sticky posts. Dimensions 350×246 px (cropped if larger, distorted if smaller).
The Morning After
a) Shows up above title of latest post on homepage. Maximum width 470px (downsized if larger).
b) Square thumbnail at beginning of “featured post/s” on homepage. Size customizable.
Triton Lite
a) If set to a sticky post, shows up in the slider of the theme. Dimensions 950×270 px (cropped if larger, no show if smaller).
b) Thumbnail in regular posts, on main and category/archive pages. Width 290px (resized to fit).
Twenty Eleven, Duster
a) Replaces regular header image when viewing single post or static page; Width 1000px (downsized if larger, no show if smaller).
b) Shows up to the right of the “Featured Post” title etc. when you use the special “Showcase” template of the theme. Maximum width 500px (downsized if larger); or full width (1000px) if you set your Large option in Settings>Media to 1000 and the image is 1000px or wider.
See my post The workings of Twenty Eleven / Duster.
Choco, Liquorice, Paperpunch, Titan, Vigilance
Thumbnail at beginning of post (on main posts page, archive pages, and single post), so practically useless.
Choco: maximum width or height 111px; Liquorice: width 55px; rest: width 250 px (all downsized when larger).
Notes:
• To be more precise: cropped if/when larger actually means cropped when larger and in a different aspect ratio.
• Update February 2011: The Support doc on featured images has been rewritten to include a whole new section (“Other Featured Images”) after relevant posts of mine in the wp.com forum; but the introduction is still misleading.
• Related post: On the recent featured image issues.
https://wpbtips.wordpress.com/
wordpress tips 
dear panos,
did you have any social networking ? cause i wanna ask you something,, but i dont know where i must write xixi
soo, i write here XD
If i wanna post a new post in my WP, why the visual mode its nothing ? where it is ? how can i get it back ? please tell me ^^
thankss,
Posted by Sohwaa | August 24, 2010, 21:50@Sohwaa: No I don’t have any social networking. This blog is about tips and advice, so this is where you ask me!
Go to Users > Personal Settings and make sure the option “Disable the visual editor when writing” isn’t ticked.
Posted by Panos | August 24, 2010, 23:59Thks for the tips !
Any way to crop the featured image on the frontpage ? Forcing w/h in style.css doesnt work for me. Im under TwentyTen.
Posted by zqzq | September 10, 2010, 08:23You’re welcome. But I don’t understand the question – please clarify! (The blog front doesn’t display a featured image unless you’ve set it to display a static page instead of your latest posts. And a featured image larger than the appropriate for the header is automatically centered and cropped. Also, are you talking about a wp.COM blog? Better link to the blog in question.)
Posted by Panos | September 10, 2010, 11:37Sorry, I should give the details :
_ For sure, I got a static page on my frontpage.
_ Im not on a wp.COM blog, while it is on development, Im working offline.
For example, my header is 940*130 and my image the same size.
If I crop my featured image to 500*130, it doesnt work, the featured image still 940*130.
Posted by zqzq | September 10, 2010, 11:55Well, yes, the header image area has fixed dimensions, and a featured image is supposed to adapt to them by being stretched, if smaller, or cropped, if larger.
I don’t really see why you’d want a 500px-wide image in a 940px-wide area, but anyway: do you want 500px-wide header images in general, or 940px-wide header images in general with just one particular image at 500px? In the first case the CSS (in wp.com at least, and assuming you want it aligned left) would be:
#branding img { width: 500px; }
In the second case I would leave the image at 940 and edit it to delete the useless part instead of cropping it (leaving 440px of white space).
Posted by Panos | September 10, 2010, 13:24Thanks for the answer. I wanted to know if it was possible to auto-crop it, and your answer is no. Do you have any tips article about designing the excerpts and full posts ? Thanks again.
Posted by zqzq | September 10, 2010, 16:41@zqzq: You’re welcome. If by “designing the excerpts and full posts” you mean via CSS, then no: my articles are almost exclusively about the free versions of wp.com themes.
Posted by Panos | September 10, 2010, 21:52Does it take really long time to upload the feature image . Thanks
Posted by sahadevh | October 23, 2010, 12:19@sahadevh: No, a featured image is just an image like any other; it doesn’t take long to upload unless you’ve got a browser problem.
I visited your blog when you posted the comment, and you had no featured images at all. I visited it again now, and now you’ve got one. So you were probably doing something wrong or missing some step but apparently figured things out in the meantime.
Posted by Panos | October 23, 2010, 20:23This comprehensive and detailed list is an excellent resource. Bookmarking! :)
Posted by timethief | April 3, 2011, 05:00Thanks! I posted the link to this post in the forum because the support doc isn’t up to date. (And that doc used to be a lot worse: the whole “Other Featured Images” section was added long after I published my post.)
Posted by Panos | April 3, 2011, 05:43Dear Panos,
I am using: The Morning After theme on my blog. I looked at the demo you can see it has thumbnail images on the featured post with a brief text summary of the article, and clicking “continue reading” takes you to the full article.
My query is this: is there a way to have the same image as the thumbnail on the homepage but bigger in the post. I’d like the homepage to remain the same with the thumbnail image with some text next to it, but when you click continue reading I’d like the image at the top of the post page to be the full width with all the text underneath it. Is this possible?
Posted by Anonymous | April 9, 2011, 21:35How can I do this?
Posted by Anonymous | April 9, 2011, 21:35You upload the image, set it as the featured image of the post, then also insert it as a regular image the normal way.
Posted by Panos | April 11, 2011, 16:58I tried an only got the featured image on the post. What do you mean by also insert it as a regular image the normal way? Do you mean to insert it on the same page as the featured image? I tried that and it did not work. Where do you but the regular image?
Note: I have a more tag link.
Posted by Anonymous | April 12, 2011, 02:44Link to the blog in question please.
Posted by Panos | April 12, 2011, 06:35Figured it out. :) Thanks anyways
Posted by Anonymous | April 12, 2011, 22:38Never mind… Here it is:
http://hypertextinfinity.wordpress.com/2011/03/11/support-japan/
On also check it on the homepage.
Posted by Anonymous | April 12, 2011, 23:52Yes you figured it out, only now you’ve got both the thumbnail and the fullsize image on your homepage. Go to Appearance > Theme Options > General, uncheck the option “Check this if you want to show the full post content on home page”, click Save Settings.
Posted by Panos | April 13, 2011, 10:59Thanks a lot! You really helped me!
Posted by Anonymous | April 13, 2011, 23:57@hypertextinfinity: You’re welcome!
Posted by Panos | April 14, 2011, 12:07Hi there,
i am using Twenty Ten but my posts are quite long and its starting to look messy.
So i want to change to something
1. (e.g. Rounded looks nice) where each post is inside a another coloured box. (not sure if you udnerstand, like in rounded, each post is inside a box, so that there is a space of different colour between 2 posts.
2. I owuld like the posts on my home page to be summarys with a picture (any picture from my post) how can i do that? an e.g. is the summary style on http://www.michellephan.com
Thanks
Posted by lilyandtom | June 11, 2011, 11:01Hi,
Messiness apart, too many images etc. on your main page may also mean slow or incomplete loading (for a visitor with a slow connection).
You can stick to the theme you’re using and just decrease the number of posts per page (Settings > Reading), as well use the read-more tag to truncate the posts:
http://en.support.wordpress.com/splitting-content/more-tag/
You can do the same with any other theme. But if f your main concern is posts more clearly separated from one another, I can’t suggest one for you: you’ll have to select one you like, by previewing them in your dashboard, or browsing here:
http://theme.wordpress.com/themes/
You can also switch to a theme that displays featured images (see post above), or to a theme that displays custom made excerpts on the main page (see my post on the Excerpt function).
Posted by Panos | June 12, 2011, 19:50thanks for this post. i have been around with wordpress for a long time yet i do not know about these stuffs. ill try these out as soon as i have the time.
cheers!
Posted by cheesecake | October 11, 2011, 17:15You’re welcome, but the theme you’re using doesn’t support FIs.
Posted by Panos | October 12, 2011, 02:27yep. it is really hard to find a theme that has all you want (or need). maybe that’s what the CSS upgrade is for. :D
Posted by cheesecake | October 13, 2011, 16:19It’s practically impossible to find a theme in which everything will be the way you would prefer. But I find that reasonable, since it’s a free service: in my opinion, they already offer a lot.
The CSS upgrade is relevant in part only: CSS editing allows you to change the appearance of a theme (fonts, colors, widths, etc.) but not the way it works. If a theme doesn’t support featured images, the CSS upgrade cannot change that.
Posted by Panos | October 13, 2011, 23:24I tried featured image for one of my post. It worked all right but after a day or so it wasn’t there anymore when I viewed the post. You have any suggestions why? I use the theme coraline. Thanks for your help.
Posted by Bites Of Sweet And Spice | November 23, 2011, 07:52First see here:
https://wpbtips.wordpress.com/2011/10/21/on-the-recent-featured-image-issues/
If you had successfully tried a featured image before I published that post, then the post answers your question. If you had successfully tried it after I published that post, then you must link or point to the post you’re talking about.
Posted by Panos | November 23, 2011, 17:30I guess it’s because I resized the image slightly smaller than the required size to make sure it’s not cropped. It’ll work only if its of exact dimensions or larger from what I understand.
Posted by Bites Of Sweet And Spice | November 25, 2011, 17:40Yes – see here:
https://wpbtips.wordpress.com/2011/10/21/on-the-recent-featured-image-issues/
Posted by Panos | November 25, 2011, 19:39It’s also mentioned in the note at the end of the post: “images in different dimensions will be cropped (if larger) or not show up (if smaller)”.
Posted by Panos | November 25, 2011, 19:40