Ummm well i say phtoshop and just go do some tutorials once youve read enough of them you know where everything is in photoshop and you can start to make half-decent ones without needing a tut.
I've never used GIMP but ive heard its like comparing windows movie maker to sony vegas 9 (or something along those lines)
Gimp is more easy for simple animations and photo-manipulation..
But PS has many more features, however the interface can be confusing to a beginner..
-I suggest Gimp.. or you read a few tuts from out on the net