Most image processing programs like PhotoShop or Paint Shop Pro will allow you to easily create seamless tiles from most any image, or you can download a prepared background from one of the sites listed in our Background Images list . Be sure that your text and other images contrast well with your chosen background. Change your monitor's brightness and contrast adjustments to check the page as other people may see it. A visitor doesn't see anything happening while a background image downloads. The smaller the better.
Background images can be either GIFs or JPGs.
Consequently you can also get very creative with them.