<?php
if ( is_singular() && has_post_thumbnail( $post->ID )) :
   $thumb_id = get_post_meta($post->ID, '_thumbnail_id', true);
   $thumb_url = wp_get_attachment_url($thumb_id); ?>
<style type="text/css">
#primary { background: url("<?php echo $thumb_url ;?>") no-repeat scroll right bottom #FFFFFF; }
</style>
<?php endif;
?>