The top of the Matrix page is only 9 kb as a GIF. This is because the large gray and green surfaces are uniform in color, and because the image as a whole uses a very low number of colors.

As a JPG saved at quality-level 6 (medium/high) in Adobe Photoshop, the same image is 17 kb, nearly twice the size of the GIF file. Of course, 17 kb is still not a very large file, and if you had only this one image on your page the difference in download time would be insignificant. But when you start piling on graphics on in the form of navigation buttons, bars, and the like, download time will increase rapidly, not to mention unnecessarily. Keen observers will also notice that the white letters are brighter in the GIF version, that the green around the letters in the top line of the JPG version is uneven, and that the whole JPG image just looks less crisp and attractive than the GIF image.