I’ve been meaning to add a gravatar plugin for months now.  I finally got around to it today during a 3 hour break between classes.  This is an explanation of a gravatar (from Wordpress):

Gravatars are Globally Recognized Avatars. An avatar or gravatar is an icon, or representation, of a user in a shared virtual reality, such as a forum, chat, website, or any other form of online community in which the user(s) wish to have something to distinguish themselves from other users. Created by Tom Werner, gravatars make it possible for a person to have one avatar across the entire web. Avatars are usually an 80px by 80px image that the user will create themselves.

A Gravatar is essentially the same thing, but they are all hosted on a single server and are called up by encrypting the users’ email address via the MD5 algorithm. So instead of having one avatar on one forum you visit, and another at a blog you visit, you could have the same avatar at both.

Basically, you can choose a picture for yourself that is associated with your email address.  Whenever you post a comment on my site (or another site), with that email address, your avatar will display.  You can sign up for a free gravatar at: http://site.gravatar.com/