The Gimp is a freeware program which is equivalent to Photoshop. It's an awesome program, the only downside is the help button has never worked. That's not too much of a problem though, because the Gimp has a huge network of people online who can give advice and help you do whatever you're wanting to do with your picture.
Here's the link to download it:
http://www.gimp.org/downloads/
The Gimp is compatible with Windows and Linux, Mac needs a plugin to operate the program
