[Post re-written December 2011, after the recent WP updates.]
Profile & avatar
Normally you upload your avatar image in Users > My Profile; in case this doesn’t work, you can do it directly at http://en.gravatar.com/.
Note: avatar or gravatar = user pic; not to be confused with favicon or blavatar = blog pic (shows in the address bar of your browser; uploaded in Settings > General).
You can enter some brief info in Users > My Profile > “About you”. But you don’t have to, and in most themes it won’t show up anywhere. In the following themes, however, if the About you field isn’t empty, your avatar and profile info will show up below each post (on the single post view only):
Chateau, Choco, Dusk to Dawn, Duster, Elegant Grunge, Enterprise, Imbalance 2, Liquorice, Matala, Mystique, Neo-Sapien, Next Saturday, Pink Touch 2, Selecta, Skeptical, Splendio, Suburbia, The Morning After, Twenty Eleven, Twenty Ten, Vertigo, Vostok.
Since in most themes your profile isn’t displayed, and since displaying it under each and every post is pointless except in multi-author blogs, a different way of displaying info about you is preferable or required. For detailed info, you use an About static page; for short info, you can also use the Gravatar widget or a Text widget.
Many themes display the author byline in the post metadata (depending on the theme: above, below, or next to each post); some don’t. In many cases the byline is displayed only when at least two authors have published at least one post each.
The author byline will be your actual username, unless you set a different name or nickname in Users > My Profile > “Display name publicly as”. Publicly means it’s the name that will also show up when you post comments in your blog or other wp.com blogs while logged in. (Doesn’t apply to posting in the wp.com forums: there it’s your username that will always show up.)
Note: normally your name should link to your blog. You set that in Users > Personal Settings: scroll down to Account Details, paste blog URL in the Website field, click Save Changes.
Byline displayed on main posts page as well as on single post view
in all themes except the ones listed below.
Byline displayed on main posts page as well as on single post view, but optionally
in iNove, Monochrome, Under the Influence.
(Option set in Appearance > Theme Options.)
Byline displayed on main posts page, but not on single post view
in Black-Letterhead, Contempt, Fusion, Sapphire, Tarski.
Byline displayed on single post view, but not on main posts page
in Autofocus, Chaotic Soul, Chunk, Coraline, Fauna, Nishita, NotesIL, Oulipo, Pilcrow, Splendio, Structure, Suburbia, Vermilion Christmas.
Byline not displayed
in Almost Spring, Bouquet, Chaos Theory, Day Dream, DePo Masthead, Dusk, Esquire, Fadtastic, Fjords 04, Fresh & Clean, Fresh Bananas, Freshy, Greenery, Grid Focus, Hemingway, Iceburgg, Jentri, K2-lite, Light, MLB (all three), Modularity Lite, Neat, Neutra, Pool, Quentin, Solipsus, Steira, Sunburn, Supposedly Clean, Thirteen, Treba, Twenty-eight Thirteen, Unsleepable, Vertigo, White as Milk.
• In Settings > Discussion you can select if the comments under each post will include the commenter’s avatar or not.
• In multi-author blogs it’s useful to add the Authors widget or the Author Grid widget: allow visitors to sort posts by author.