In this post from digg.com today, there's a pointer to this tutorial from Icemelon.com today about creating dynamic images in PHP.

If you frequent forums, you'd probably remember images in people's signatures that would display information about your computer. Such mysticism! This tutorial will teach you that, and more!

They show you step by step how to use the GD extension in PHP to update a pre-existing image (a PNG in this case) with the viewer's IP address dynamically. It's just a small bit of what you can do with GD, but it's a good introduction to what it looks like...