Well I found a solution. It’s not totally ideal, but it works. Insert the following into the HTML of each page (Pages > Click on a page to edit, then click on the HTML tab):
<style type=”text/css”>
body {
background-image:url(wp-content/themes/twentyeleven/images/bg/yourimage.jpg);
background-repeat:no-repeat;
background-position:top;
background-color:#000000;
}
</style>
Just make sure you change the URL to where your image is located.