Gravatar is a widely used service to include a thumbnail of a person. It's a feature often used in blogs to display a face next to a comment. Internet users upload their picture at http://en.gravatar.com/ and assign it to their e-mail address.

The application can then generate an md5 hash by using this e-mail address and can display an image of them, without exposing the actual mail address.

This procedure works with PHP like it does with any language where you can generate an md5 hash:

$gravatarHash = md5(strtolower(trim('[email protected]')));
echo "<img src=\"http://www.gravatar.com/avatar/{$gravatarHash}\" />";

Let's add that feature to the guestbook block.