i think you’re after a normal map. there is a photoshop plugin by nvidia that (oddly enough) takes a bitmap image and converts it to a normal map. for a 3d render, you need a “screen space” normal map (read: from the camera, not the object space). regardless, you can take a normal map into reaction as a surface shader and then pipe it into a layer node to take advantage of 3d lighting.
your inquiry about displacement maps is kinda either a 2d thing (like combustion’s) or a 3d thing (like flames). is that actually what you are after?
//gD