I come from a broadcast background editing on Discreet Smoke. I'm used to doing matte or "fill" keys with a foreground, background and matte. The matte cutting the hole and filling it with the foreground. I may just be stupid, but for the life of me, can't figure out how to do this on Motion. HELP!!!!
I think you may be looking for the Image Mask. It allows any channel from any object (images, text, shapes, particles, etc) to be used as a mask. You'll find it under Object->Add Image Mask. Motion doesn't ask for an explicit background, the way Flint/Flame/Inferno/Smoke do.