I can only presume you are building in tables and have never used css before ( as someone that knows css would never ask a question like that )
put something like this between the <head> tags
<style type="text/css">
<!--
body {
background-image:url('back.jpg');
background-attachment:fixed;
}
-->
</style>
back.jpg needs to be the root pointing to the image, for example
background-image:url('images/back.jpg');
if you could post the site to my forum I can check it and do it for you ( address in sources )