Another option is film-gimp, aka gimp-hollywood, which is reputed to be the second most commonly used image editor after photoshop. Apparently it has now been renamed CinePaint and as of May 2006 is in the midst of some kind of upheaval. It is also based on GTK-1.0 (this makes it kind of clunky looking). It is scorned by the mainstream gimp developers for some obscure technical reasons (or maybe not so obscure political reasons), and so has been forced to move out into the world on its own.
On a pretty much unrelated topic, take a look at inkscape which is a vector graphics editor with features supposedly similar to Adobe Illustrator.
Last, but not least, let us not forget image magic which is so simple and powerful and easy to use. And it can be invoked off the command line allowing you to use it in scripts, or run it from web pages (PHP scripts) and so on, my hat is off to these folks.
Adventures in Computing / tom@mmto.org