Gimp is a great program at a great price.
I found it very difficult to use, however.
I much prefer
http://www.paint.net/This program also is free and open source. There are tons of great ad-ons and a good bulletin board for support. You can always see what you are doing. It is very simple and intuitive to work with layers. It generates preview effects over the entire image instead of a tiny preview window. If I recall, Gimp has a few things that you just have to try to see what they do, and then undo, adjust, apply again, etc.
I used The Gimp in my lab for a couple years (I teach computers in a K-8 Catholic school). My 8th graders had a hard time getting anything done with it. It wouldn't save to a network account; they had to save to the hard drives. The guides, were difficult to use. Now we use Paint.net; I'm/we're having a much easier time.
