I am looking to achieve a similar effect to this website i found: <http://www.august.com.au/>

When on this website if you zoom in/or out ctrl +/- the background doesn't change. Only the content zooms. 

I want to be able to set a background image for my website and have it so that the background image always fills the screen regardless of the screen resolution or zoom options. 

I understand that you can use the width:100% to make it fill the page, however my understanding is that this is applied as the page is loaded and if the user zooms after loading the image will not re-sized. 

Does any one have any suggestions on where to start with this one? I have tried search the web for information but wasn't able to find very much. 

Also i am guessing this will be achieved using something like JavaScript and not css??