Actually, when loading images I always use what might seem to be a crappy format: 72 dpi .jpg. However, you have to remember that MG processes the image itself to fit whichever rack format and width it needs, so large files could actually be more problematic for MG itself, resulting in the rescaled image looking off while the module page looks better. Try giving the site something smaller to chew on and see if that works.
Counter-intuitive, yeah...but it seems to do the trick.