CSS Background Image Help!

[Reece^B]

Hi Folks

I'm going to pickle your brains here, I've set a background image in additional.css with the following code on my dev site:

#contentMain {
background-image:url('images/styles/DeFraction/old_wall.png');
background-repeat:repeat;
background-attachment:fixed;
background-position:center;

This gives me a background image, but other parts of the style are white (CMS/Articles etc...)
So in style variable editor I add background-image:url('images/styles/DeFraction/old_wall.png'); which turns the white into images however it doesn't look right when you test.

As you can see when you scroll it's clear there's 2 images.

So I need advice how to create 1 big background image that everything adopts.
This site has what im after for example.

Thanks in advance