Background independent from Flash? How? (Example in topic)

[helious]

I would like to create a flash page where a background image fills out the whole browser window, and reacts proberly when the window is resized.

Here is what i mean http://www.ogco.dk/

How do you make that? Is it something in the HTML, or Flash?
Try resizing the window, and see how the Flash and Background reacts

Thanks in advance