In case you wonder why some of the posts in the “Comments” section of my posts have nice pictures of the poster… these pics are gravatars or “Globally Recognized Avatars”.
How does that work? The blog calculates a md5-hash of the email address you enter as part of the comment and looks that up at http://www.gravatar.org/ . If you have an account there and combined a picture with your email address, the picture will automatically show up next to your name in the comment.
This is the URL that uses the md5sum of my own email address – clicking on it will retrieve my picture: http://www.gravatar.com/avatar/caef891534164c2e3fee7e102a39cc97. Using a hash instead of the email address allows you to enter your email address for the display of your gravatar, at the same time preventing other visitors (and spam harvesters) to find out about your email address.
Most blog software supports the use of gravatars. You can even have multiple images, and assign ratings to them so that “family-safe” web sites will not suddenly show shocking pictures of yourself.