What is a Gravatar?
Gravatar is an abbreviation for globally recognized avatar, the concept of which was invented by Tom Werner.
On the Gravatar website, users can register an account based on their email address, and upload an avatar to be associated with the account. Gravatar plugins are available for popular blogging software; when the user posts a comment on such a blog that requires an e-mail address, the blogging software checks whether that e-mail address has an associated avatar at Gravatar. If so, the Gravatar is shown along with the comment.
Source: Gravatar - Wikipedia
The recent release of WordPress 2.5 included built-in support for Gravatar but implementing it on your blog requires a few additional snippets of code to the comments.php file.
The code structure of your comments.php file will probably differ from mine but searching for the correct position shouldn’t be too difficult.
Via Add Gravatar Support to Your WordPress Comments | Theme Lab
