I think we'd need to see what you are working with so that we can test things out. Do you have a link to the site in question?
It's hard to visualise exactly what you need and of course will depend on your structure. If you need a background on top of an existing background image then you can't apply it by a div that is outside. You would need to have a nested element with the new background in place so that it would sit on top.
Perhaps another solution is the absolute overlay technique. You set position:relative on the parent of the element holding your image and then you create an absolute overlay to which you apply your new background image. Using co-ordinates of top, right, left and bottom will ensure the absolute element stays track with the static element but of course you will need to control stacking levels if other content is in the mix. (The middle section of this page uses the absolute overlay technique to create a section that stretches from a point below the header to a point above the footer (and can't be done any other way). Although the application is different to your requirements the technique is the same.)
It may be easier if we see what you have already before suggesting complicated fixes as they may not be appropriate and something simpler used instead.