My b2evolution Version: 4.x

Can someone tell me how to repeat my background image down to fill the screen even if the content doesn't go that far? I tried repeat-y but that seemed to make matters worse. Basically, when you look at this page: http://desrosiersconstruction.org/index.php/Portfolio/, if you have a large monitor, all the space below the main content area and footer is white instead of having the background image extend down. How do I fix this?