I love the movies, but something just hasn't sunk in.

I have a bitmap filled shape with no outline. I want to color on top of that bitmap with black or brown so as to make it look as if it is curled. I want to shade with precision in whatever area I want to perhaps give a folded look or a different curl orientation, whatever. The idea is to add 3D depth to it with light and shadow so it looks like slightly curled or folded paper.

The question is: Whatever black fill I put on over the bitmap just obscures the bitmap completely (and applying a transparent gradation of black just makes the bitmap transparent as well); so how do I do the most basic and most used (for realistic) illustration technique of adding subtle area shading over what's beneath? For me, demonstrating a little section of the cool microscope illustration would have been way better than the desert bitmap example in the "Create an Illustration" disc movie.