I suspect that may be why some people are reporting weird scrolling.
I know we have them on top of each other. This morning I tried it by applying it to <body> but future adverts will use transparency over the <body> background. The user I had confab with still reported the same issue.
Besides, placing one background over the top of another should have no issues at all. Just like the header is #div with a background, on top of #wrapstar, which is on top of <body>.