Background Images:

"It is easy to get carried away with background images and plaster them all over your web pages. Some visually hyperactive people might believe it looks good to have a full-on brightly coloured photograph tiled across the background of a page, giving the user a Mensa-esque challenge in deciphering the foreground text. This is an extreme example, but the fact is that the most user-friendly, readable text is black on a plain white background or white on a plain black background (there is also a suggestion that a slightly off-white or off-black background is better as this reduces glare).

So, the best use of background images is either to use them where there will be no content over the top or making the background image very light, which would also reduce the file size of the image, because there should be less colours involved (supposing you are using an indexed-colour format, such as GIF)."