I don't think that Poser stores the images in memory, because if you render, then alter the map, save it, and re-render poser updates it. So it must draw the file in when rendering. What may be stored in memory is the location and maybe the low-rez version you see in the texture display mode. When rendering poser will draw in the bump map, trans map, textures, generate the shadow maps, etc. That is where your memory is being chewed up.