In Poser1 days Computers were very slow (I think on Mac it was just the age of Quadra?) and the conversion took so much time that it was clever to do it only once and save the result in order to directly load it when needed. It should have been changed with Poser3, but somehow it just sticked to the BUM writing. As a matter of fact, the bumpmap filter which I wrote for GraphicConverter takes a couple of milliseconds on a G3 (whis is slow as well nowadays... :-), so PPP simply converts it "on the fly" now for every render.