The only element hanging out there at the moment is the body.
So you can hang a background image off the body and align it left or right starting from the very top or the height of banner so it starts at the top left corner of the white area.
I seem to remember using the html tag as well to hand a background image off - so try that out too, if it works in modern browsers then you have two elements that you can apply background images to.
The elements are overlapping so give body a transparent background-color as well so that you can see html's background through it.
With the web developer toolbar you can select Display > Element Information
Then click in the area you are wanting to know about and it show all the elements and parents of the element in the spot you clicked.