JPEG has better compression, which makes it easer to download, but it is a lossy compression scheme and can distort the image. TIFF is not lossy, but it doesn't compress well, resulting in larger files and longer download times. However, both, when loaded into Poser, use roughly the same amount of RAM (for identical resolution). Not sure about the BMP format--I happen to prefer PICT, which is my platform's native image format since PICT will allow me to reduce the memory demands of a texture by reducing the bit depth of the texture. (Reducing bit depth to 8 bit colors is an efficient method to reduce the memory load on Poser for simple textures and transparencies--early Poser documentation actually suggests going this route to speed up renders and get the most out of your RAM.