You may be noticing the “Gravatar” icon now showing up in the comments, while some commenters have a picture or avatar in its place. A Gravatar is a globally-recognized avatar. Getting one is easy. Set up an account at gravatar.com, add the email addresses you use when you comment, and upload an avatar.
Once you have a Gravatar account, any time you comment on a blog that, like this one, is Gravatar-friendly, your Gravatar will show up. No need to register with the blog, just use one of the email addresses that Gravatar knows about.