Herself's Webtools

When there’s been time I’ve been learning a bit about computer graphics, I use ‘The Gimp’ for still art, photo editing and animated gifs. Gimp is open source under the GNU license, so it is free for you to download and use. There are versions for Linux, Windows, and OSX.

It will take a week or so curled up with your computer and the manual to really get
the full use of gimp, but it is time well spent.
GUM ( Gimp User’s Manual )

All the graphics on my sites are done in the gimp.

More information:
Tutorial Blog – GIMP tutorials

See also Gimp Basics: Beginning

NOTE:
If you are installing Gimp on 10.5.1+ you will need to download and install new X11 and Gimp. Detailed directions are here