You still don't need to go to those lengths to achieve this... it's way over the top to be adding that field and then having to maintain the override in line with gantry and template updates. Just do it all in custom CSS. You can add the other attributes required to make the repeating image.
#g-page-surround {
background-image: url("yourimageurlgoeshere");
background-repeat: repeat;
background-size: auto;
}
If you are having trouble with that - post a link to your site and I can check the CSS.
Regards, Mark.