Pngnq quantizes 24-bit (RGB) and 32-bit (RGBA) PNG images down to 8-bit (palette) PNG8 images. http://pngnq.sourceforge.net/ See http://www.sitepoint.com/blogs/2007/09/18/png8-the-clear-winner/ for one motivation on using PNG8 images (IE6 support). Another motiviation is that PNG8 images are smaller and do a good job of lossy compression of screen shots.