On this site and a lot of sites around the web, if you leave a comment on something posted and provide an email address, a picture of some little computer generated monster or geometric shape or other graphic will be generated to be used as your avatar.  You can have control over what picture appears for your email address.  See my quick little tool that shows you what avatar appears for any email address.

gravatar.php

For example when you see a comment from my email address, you will see my gravatar: