Stick background to content

I would like to know how to make the custom background dynamic fixed :-) Let me explain.

I’m creating a homepage with a fixed background where the middle of the background is transparent. The image is “cut in half” and you can only see what meant to be seen, however when you zoom in or out it’s fixed as it should be but I want the “two” images to stick to either side of the boxed layout not seeing some grey content behind the box.

Is that possible at all?

I’m making a private comment with the URL to the homepage.

Thanks in advance :-)
Valdimarsson