GIMP is a wonderful program, filled with all kinds of good stuff! I found it much less user-friendly than photoshop, though. I have a feeling once you learn it, it would be easy enough to use, but I feel like it's a less intuitive program than photoshop. Then again, I learned in photoshop, so of course that would be the most natural for me to use
If you can't afford photoshop, though, GIMP is my next favorite!!
You really can't go wrong with it! Just make sure you download it from a site that has already patched the errors