SmushIt is an online tool that helps you reduce the size of your pictures without making them smaller or losing quality. I previously showed you how to use PNGCrush, which is limited to PNG files. SmuthIt works with all popular image types.
SmushIt even comes with a Firefox extension, which will let you compress pictures when you are using Firefox. SmuthIt will also let you batch process images so you can compress more than one at a time.