There are TWO ways to get background
-Fractals(made with apophysis), you can google it to look for packs
-Gradients(I am not sure if they are on GIMP)
Anyway You do that.
First off you have to get a render, apparently you don't know what that is. A "render" is pretty much a C4D and your photo/character on the signature
Next you need to background. Which is a Gradient or Fractal.
After that.
You make it work Lulz.
I use Photoshop so...
Anyway those are the base ingredients
http://planetrenders.net/ Here is where you get a "render"