Which image format should you use? The two factors to consider are file size and image quality; you want to keep the file size down for quick viewing while maintaining the best clarity. In general photographs should be jpgs, and graphics - solid blocks of color (non-photorealistic images) should be gifs. The best way to demonstrate this is visually:
Use .jpg for photos...
.gif: 5.51k
.jpg: 4.94k
Use .gif for graphics...
gif: 1.68k
jpg: 2.17k
There is an up-and-coming format, PNG, which is only supported by the leading browsers in version 4 and above, and it's beginning to replace .gif. Call me old school, but .gif works perfectly fine for Register.com, so we aren't planning to switch for now.