Images must be RGB and I try to keep it at 100dpi resolution so as not to hog to much memory. A JPG is probably the best format to save image files in here. Hope that helps.
Whats your connection speed? I've used a dial up connection to view an none load! lol they are all to big. Perhaps that is the problem. Hover over the thumnail and you'll get some details including the images file size. Worth a try.