Do you BMP or BUM? The latter is a "processed" format and although you can open it as a BMP and save it as JPG, Poser will not use the resulting file correctly: if you don't have the unconverted image from which the BUM was made you will need to stick with it (there is a back-conversion technique, but it is imperfect I believe). If you just mean textures supplied in BMP format then ignore me.