Always view your pages on as many different platforms, browsers, and browser versions as possible. To cater to a heavy browser base of Netscape users, use Victor Engel's 216 color Netscape color cube as the palette when saving or creating an image. Photoshop Edit originals in RGB mode in Photoshop, then change to Indexed Color when finished editing. To help out those with 8-bit color, pull all images that will be on one page into one file in Photoshop. The Previous palette is the key and saves all of the images with the same palette of colors.
Image compression decreases file size.
Some exceptions should be noted. Take great care converting GIFs to JPEGs. In Photoshop when converting to GIF, adaptive is usually the best palette option. In Photoshop when converting to GIF, diffusion dither is usually best for photographs and none is usually best for illustrations. When saving as JPEG, low quality is a good starting point because there is usually not that much difference between it and medium quality.