that was from so long ago, I don't think I still have.... I was just playing with the idea at work, and still had that .jpg. When I did this background, I looked for photos of landscapes that matched perspective in areas, then overlayed them in Photoshop and painted a layer mask to blend together.
I was reading a book on matte painting at the time [it is #9 in your Amazon wish list]. (in it they were painting on a render, then projecting the painting back onto the model as a texture). It looked very effective: the example in the book was the Jedi temple from one of the Star Wars movies.