By now you have read the Announcement.
What is a gravatar?
A gravatar, or globally recognized avatar, is quite simply an 80×80 pixel avatar image that follows you from weblog to weblog appearing beside your name when you comment on gravatar enabled sites. Avatars help identify your posts on web forums, so why not on weblogs?
Stay up to date with The Gravatar Weblog
How do I get a gravatar?
Signing up for a gravatar.com account is FREE, and all that’s required is your email address. Once you’ve signed up you can upload your avatar image and soon after you’ll start seeing it on gravatar enabled weblogs! Sign up now!
How do I get gravatars on my weblog?
Setting up gravatars on your weblog is easy, you don’t even need an account! Plugins are available for leading weblog software, and our tutorials will have you running gravatars in no time. To request a gravatar from our servers, you simply add an image to your comments area with an “src” attribute that points to our gravatar image generator and includes an MD5 hash of the commenter’s email address. Since all gravatars are rated with an MPAA style rating, you can restrict your site to show only gravatars whose content you are comfortable with.
Learn more at this link.