After you’ve selected an avatar, there’s still more to do on your profile page.
If you click on the Profile tab on your preferences page:
then you’ll have several fields you can fill in, including an “About me” section, website, more background images, and your date of birth.
The two background images are for your profile background (similar to the banner image on Twitter or Facebook) and your user card, which is the image that pops up when people click on your avatar in a post:
If you fill in your profile all the way you get the Autobiographer badge as a reward.