Hi, i'm having a problem with the correct positioning of a background picture in the body of a forum i'm customizing now. Here are some display pics:

This one is in full screen and so the background picture falls in place with the elements okay:

Now this one presents the problem - the browser window is restored down and is ~800-900px in width:

Now as you'll notice if you open the page fullscreen and are above 1280px in width resolution, the background picture is okay and in sync with the rest of the elements. But if you restore down your browsers window and dimension it to below ~900 px or even drop to a lower display resolution than that you will notice that the background pic is off sync with the rest of the elements.

You'll notice how the upper third (the background picture) stays centered, while the other elements are shifting to the right. How can i fix this? How can i make the background move alongside with them?