If you have JQuery loaded already, you can just do this:

$('body').css('background-image', 'url(../images/backgrounds/header-top.jpg)');

EDIT:

First load JQuery in the head tag:

<script src=//ajax.googleapis.com/ajax/libs/jquery/1.7.2/jquery.min.js type=text/javascript></script>

Then call the Javascript to change the background image when something happens on the page, like when it finishes loading:

<script type=text/javascript>

    $(document).ready(function() {

        $('body').css('background-image', 'url(../images/backgrounds/header-top.jpg)');

    });

</script>