I’ve begun using PNG images rather than JPEGs and GIFs on this site. For large, full-color continuous-tone images (such as photographs), PNGs are larger than JPEGs, but have a number of advantages over JPEG, including the fact that image quality is superior (not that any of my images are that great to begin with).
I found a great tool for compressing PNGs. It runs under a variety of OSes and provides an easy way to make PNGs smaller. Because it is a command-line utility, you can quickly process entire directories at once — without all the tedious opening and saving as with a GUI.
On my images it provided a savings of about 8% on large images, and nearly 20% on small images, as compared to PNGs created by Photoshop 6.
The utility is called pngcrush.