A displacement map is just like a bump map (a grayscale image). Haven't really used them myself but i assume the workflow is really similar to the bump map. Just that you need a pretty high number of polys for it to apply correctly, as it will actually change the modeling of the object. Sorry i can't be of more help but like i said i haven't tried displacement maps yet myself. You should check maya's help manual, search for some tutorials on the net or ask someone else here on the board. Hope it turns out ok in the end .