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.
To know more about this, click here: