The IBM developerWorks site has a new tutorial posted (registration/login required) talking about image creating in PHP using the GD library.
The purpose of this tutorial is to show you how to get started with the GD library and to provide a variety of techniques you can apply to your Web-page coding. The tutorial doesn't cover every GD function, but it teaches you the basics. The Resources section provides Web sites where you can learn more about using the GD library.
They go through the creation of sample images, working with gradients and lines, adding text, saving the image data out to a file and how to work with existing graphics to modify them.