Here was the function I used

    function mytheme_preprocess_page(&$vars) {
        if (isset($vars['node']->nid)
        {
            uri = $vars['node']->field_background_image['und'][0]['uri'];
            $path = file_create_url($uri);
            drupal_add_css('body { background:url(' . $path . ') left top no-repeat !important; }', 'inline');
        }
    }