You will have to open up your index.php file and look for:
<body style="background: <?php echo $s5_colorback; ?>;">
Then replace it with:
<body style="background: <?php echo $s5_colorback; ?> no-repeat center top;">
I found the index.php file in templates folder.
Then edited using dreamweaver by 'find and replace'.
Then uploaded it to my site.
But still the image is repeating.
Please see the screen shot.