the short answer is this:
<body style="background: white url(yourbackgroundimage.jpg) repeat-y;">
You could put the style in the head though, or even in an external style sheet.
You could even get fancy with it and do this:
<body style="background: white url(yourbackgroundimage.jpg) repeat-y top center;">
That would ensure the image is centered horizontally. Probably only useful if you're working relative to the center of the browser window, but if you're looking to divide the page in half, that'd do it.