[SOLVED] Background image scrolling with main body

My wife wanted her website header image to be full-width with the body fixed at 900 pixels. By following the advice found in other threads, I managed to achieve that as you can see:
http://www.domni.com/TypeInsights2/

However, the header stays fixed in place when the user scrolls down. How can I make the background scroll in sync with the body?

Thanks,
Robin