Gimp is really a cool program, and it is free !
See the official
Gimp web page.
I recommend that you'll have a look at tigert's superb pages, and his very useful gimp tutorials.
A nice feature of Gimp is that it is extendable, you can write PlugIns for Gimp. See the PlugIns registry page!
You can also write scheme script, like those I wrote: and
Some work I've done thanks to Gimp: