Notice how the background slowly changes colour over time. Refresh the page a few times - you will see that the overlaid image moves around as well. This creates a unique effect on every page load, meaning that you will never have exactly the same design element twice. Leave us a comment to show how you have implemented this effect into your designs - we'd love to see how creative you have got!
The below effect was achieved by adding a third div inside the 'id="overlay"' containing div. Giving it absolute positioning as well means you can then use a transparent png image in the foreground of this third div. Obviously, being transparent, you can see through certain areas of this .png and on to the background we have created.