If you are using a background image for a faux column effect in a fixed width layout then you don't need any of those negative margin or nested wrapper techniques.
Unless I'm misunderstanding what you want I'm not sure why you need to complicate things if you are going to use an image anyway. All those negative margin techniques (which are loosely based on my original technique (which was largely based on Doug Livingstone's original idea posted in these forums about 7 years ago)) are methods for creating equal column colours without using images. If you are going to use an image anyway then use a simpler layout.
Just float the three columns with appropriated width contained in a 980px wrapper which will hold your faux column image. Something like this very very old example.