OK so I am trying to learn....stock photos are what you usually use behind the main image, kinda on top of the background...right?
Yeah, or as the background....
There are actually pretty much endless choices for it.
After the sig contest, I will really try to get a tutorial up on it cause the sig I made used nearly no brushes, but nearly all stock photos and adjustment layers. That should help some people out.
Anyway, practice with finding a photo close to the color of the render/sig that you are doing and then set it to linear dodge, screen, etc. Just play around with it.