Can someone tell me how Gimp works and how do you make crossovers with characters from different movies?

I'm a beginner when it comes to making icons and I'm going to use Gimp to make them, but can someone just give me a basic lesson on how it works, I read the maual on internet, but I don't understand it quite well (I was going to post one of my icons, but since I'm in school I can't do it, but I can do it when I come home)! Could you also please tell me your favorite out of my icons that I have made so far and why it's the best, maybe your least favorite too and what I can do better!