This template is not designed for a background picture. Adding something like that would require custom programming; we do not provide these kinds of services as part of our support.
I managed to get a background picture without too much trouble. not sure how it looks on other devices. What I wasnt able to do was get the white background that represents a page.
The only problem I had with the picture is I set it to repeat and it looks a bit funny when scrolling down and you see the top of the picture again