not the sub zero one i made that background and that bardock i used from what i learned just threw a few touches to it and the lighting is a brush i have for gimp and the morrision one background i also made those are not renders
Suggest reading up some tutorials in the tutorial section (as well as following them), you'll get better =)
i dont think any one here who uses gimp and i havent seen really any tutorial on here i follow ones from gimp talk but i will look
I've used GIMP. You can do some amazing things on it, but google GIMP Tutorials. You'll get a ton of results, many of which are pretty good. Add some brushing to the sig, and work on blending the render.