Hello Sara,
I'm going to give you a quick overview of the GIMP. First off, GIMP stands for GNU (which stands for GNU's not Unix, lol), Image Manipulation Program. It can do the basic things like paint, fill and rectangle select, and it can also do amazing things like super novas, clone images and map images to 3D objects like spheres and cubes. All it takes to make it work is a little bit of talent and some knowledge of the basics, and after you work with it for a while it becomes your friend.
As you use Photoshop here is a link to a tutorial for making fire effects.
BioRUST.com :: Tutorials >> Fire Effects
Have fun!
Sparky3006