Hmm....here are a few tips with GIMP that I can offer:
1) when you wanna crop a piccy for a banner....use the magnifier tool and a size 3 or size 5 brush to carefully outline the pic, then use a bigger sized brush to erase the rest of the background, leaving only the "render" itself. It's tedious and time consuming, but that's a method I found to be real effective.
2) GIMP comes out with pre-set brushes that you can use. Open a new file and practice with them on an opaque background. The Preset brushes aren't that great for use in banner-making (though you can probably use them anyway), but they are good for practice.
3) Layers are your best friend. Use the layers to segregate the pieces of your project while your working on it, like the background, the render(s), and so on. They will make your work easier by allowing you to fix one piece without messing up the other.
Soo yeah. Just a few nuggets I'm willing to share from my experiences. Also...once you've gotten the hang of the program, there are plenty of brushes for use in banner-making available in deviantArt. Just pick one you'd like for your piece, and Download it. ^_^