Prevent background image / top image from scrolling

I want to prevent the top image from scrolling down. It creates a shadowy effect of the image further down. Setting it to “fixed” in the options doesn’t do a whole lot to prevent this, so I’m thinking I need to delve into the code somewhere. But where? 🙂

Where is the placement of this image, and its subsequent shadowing down as you scroll, defined?

Viewable here: http://nice-feet.dk.linux103.unoeuro-server.com/test/