This should be obvious. If we are unable or find it difficult to read your page then we will not be staying long. Backgrounds can either demonstrate the author's savvy or bad taste. If you do decide to use a background like this, use your photo editing program to lighten it up and it can make a world of difference. Contrast is the variance between the different colors in your background.
Subtle variances mean less contrast.
The less contrast there is in your background image, the easier it will be to read the text on that background. For instance, many web site authors like to use a company logo as a background. This works great as long as you keep the contrast of the background down. Most backgrounds do not need to be very large or detailed. If you are saving your background as JPEG, use the lowest quality setting.