Here's another look at the wedding picture saved at level 5 in Photoshop. At 64 kb the file is not overly large, and the visual quality of the image is quite good. Now compare the same image below which is formatted as a GIF.

Here's a good example to show you how saving as a GIF is not always going to give you a smaller file size. This version of the photo was saved using all 256 colors of the web-pallette (at 8 bits per pixel), and because of its 113 kb file-size takes up a lot more space than the one above (and of course, it will download more slowly too). This illustrates well what can happen with a GIF file if you do not optimize it for the web by reducing the amount of colors used.

At the opposite end of the scale, this GIF has been optimized to use only 32 colors (5 bits per pixel). At 56 kb the GIF version is finally smaller than the medium-level JPG, but only by a few kilobytes, and it is of horribly inferior quality.