I've just figured out how to add an avatar to my comments on this blog. You can do the same by signing up at the Gravatar site (it's free).
You will be asked to upload an image and associate it with an email address. Use that email address when you comment, and your avatar will automatically show up next to the comment on this and other Gravatar-compatible blogs. Don't worry: Your email address won't be shown on this blog.
None of this is necessary, of course. It's just for fun. You don't need to give your email address in order to comment here unless you want to use an avatar.