here are two options:
this is going to tile the image only on the x axis
<style type="text/css">
body, html{background:url("bg.jpg") repeat-x;}
</style>
the problem would be if the image wouldnt be large enough; the other way of doing this is to create a div with z-index as 0 position:fixed and with the image as the background,
<div style="width: 100%; height: 70%; z-index: -1; background:url('bg.jpg'); position: fixed; top:0px"></div>
and dont put any content inside this div, use it only as a background div as you would with the bg image.