It looks fine on IE6, IE7, Firefox, Opera and Safari. I can't see anything wrong with your strategy.
You've just changed the code, the image isn't a background-image in the CSS in your post above.
You now have a main image at the top and another image ad-navigation-bg.jpg underneath (paler green) which is a background-image in <div id="ad-navigation"> with the links inside. This is OK.
Code downloaded to my PC will be deleted in due course.
WIN7; IE9, Firefox, Opera, Chrome and Safari for Windows; screen resolution usually 1366*768.
Also IE6 on W98 with 800*600 and IE8 on Vista 1440*900.