We could argue about the good and bad things about the different formats forever, but I'd say
Quote Originally Posted by rubah View Post
you use GIFs for images that move, jpegs for photographic images, and pngs for everything else>:[
is the general rule of thumb for final images for now (until mng takes off) For images still being processed or to edited, stick with a lossless format like png/bmp or native format of your image editor like xcf/psd to avoid quality loss - size shouldn't matter in this case

Once mng is well supported, gif should be considered obsolete and no longer used