WEll, yeah, that's true, but a .bum converted to a .bum is generally a large file in terms of MB, and will add render time, plus, it won't work as well as a grayscale image will. A Poser .bum changed to .bmp by changing the extension looks all sick green-yellow, with very little contrast between the lights and darks. When possible, it's better to use a grayscale .jpg.