Well, you see, that's the thing.
I've never done it before, but apparently people in the past have replaced textures in the game with their own, hmm...
It would seem I can make things into textures the game would accept with my pigg diver.
Give me an image you want to be the background, and I'll make a copy of it into a texture.
You can PM it to me if you like.
However, I don't see how you can add files into piggs.. this is a question I can't really answer.
Sorry. Codewalker probably knows how.