Several wordpress.com themes support what WP has called featured images. In some of these themes, setting a featured image to a post or a static page means that this image will replace the regular header image when you view that post or page. In the rest of these themes, featured images work in various other ways. See my post on featured images.
In quite a few of these themes, a featured image will display in specific pixel dimensions. So, if you wanted to use featured images, the best practice was to upload them after you had resized them to the right dimensions using an image editing application. And that’s what experienced forum volunteers had recommended from the start (see, for example, my posts on Inuit Types and Structure).
However, any image would work. Well, sort of: larger images would be cropped, and smaller ones would be blown up and/or distorted. But suddenly, two days ago users started complaining that their featured images stopped showing up. Same thing for the Modularity Lite slideshow (it doesn’t rely on featured images, but the reason is the same).
Reason? WordPress decided that cropping is ok but blowing up isn’t (because blown up images lose quality). In the only staff reply
so far ever given, this was called “a long time standing bug” that has finally been fixed:
“Actually, that has been a long time standing bug with the way we processed images. This has been fixed now. Previously, images that were smaller than what a theme specifies as the size for a featured image would be upscaled and distorted. This should never happen by default.”
Quite correct. But they should have thought of this when they started launching themes that support featured images, not a year later. And they should have informed us of the change instead of imposing it without any announcement and while they were all away for two weeks (leaving us volunteers try to figure out what was going on and help frustrated users). Anyway, the point is that from now on your featured images have to be in the right pixel dimensions (or larger, if you don’t mind that part of the image will be cropped).
For themes in which the featured image replaces the regular header image:
Check Appearance>Header in your dashboard, or consult my post on header images (the corresponding Support doc isn’t always correct).
640×392 pixels if you’re using the option to display the featured image when viewing a single post.
620×380 pixels if you’re not using that option.
(620×380 is the size of the homepage top featured image, 640×392 is the size on the single post view; but 620×380 and 640×392 are the same aspect ratio, so the image will show up in the homepage top area in both cases).
For Inuit Types:
One-column featured posts: width 600 pixels [*], maximum height 250 pixels.
Two-column featured posts: width 278 pixels, maximum height 150 pixels.
Regular posts: maximum 90×75 pixels.
[*] Although 594 pixels will show up: the remaining 3+3 pixels are hidden.
For the Modularity Lite slideshow:
Update 1: For themes launched after this post was published, see my post on featured images.
Update 2: In some of the themes that were launched in 2012, featured images will work even if smaller than the right dimensions. So apparently WP changed their mind again, or became sloppy. So apparently users are always at the mercy of the latest WP whim…