Wow, this is quite nice thegreentick! Very impressed. So to your initial question, GIMP is indeed a great alternative to PS, and is definitely worth having in your tool-box. However, for what you are looking for, I think a paint program would actually be the better choice. There are two wonderful free paint programs out there. Mypaint is a good beginner program that, from what I'm told, is easy to learn and quite user friendly. Many people swear by it. Krita is a more powerful paint program with a whole lot more options. The downside is that the learning curb is a bit steeper.

Cheers,
-Arsheesh