This plugin allows you to automatically add Gravatars for commenters to your
theme, if your theme does not already support them.

According to the Gravatar.com website, Gravatars are Globally Recognized
Avatars, or an “avatar image that follows you from weblog to weblog
appearing beside your name when you comment on gravatar enabled sites.”
You register with the Gravatar server, and upload an image which you will
use as your avatar. The gravatar image is keyed to your email address, so
that it is unique to you.

This plugin will display gravatars for the people who comment on your posts.
You do not need to modify any of your template files — just activate the
plugin, and it will add gravatars to your comments template automatically.

Credits

Based on a code snippet from Matt Mullenweg:
http://photomatt.net/2007/10/20/gravatar-enabled/
http://pastebin.ca/743979

Props to David Potter for pointing out that Gravatar normalizes email
addresses to lowercase before hashing with MD5:
http://dpotter.net/Technical/index.php/2007/10/22/integrating-gravatar-support/

Easy Gravatars

Credits