GIMP is free, and powerful, and, as omk3 said, has a steep learning curve. (I've been working with Photoshop for almost a decade, and I still get lost on very basic things in GIMP just because it's different.) You can get it at
http://www.gimp.org/ and there are help files & discussion forums; it's just... complicated.
There are no-doubt other free image-editing software programs that are simpler. But if you have to learn it from scratch anyway, you might as well start with one that has all the features you'll eventually want.