Liquid Flash - scale to browser

Hi people -

I’m trying to find a way to do something which I reckon is not too hard but I’m not sure where to start. I’d like the background (image) to fit to the size of the browser - and scale accordingly - while the foreground (menu) remains the same size.

So that the menu should not be affected by a user rescaling their browser while the image behind it is.

Could any of you get me started or throw me in the right direction?

Any help much appreciated.

Regards,
A…