Forum Moderators: open
I suggest you start with a "View Source" of your page when you access it online. This should show more of the details you are working with (or against).
From your description it sounds like they are using a z-index to display something that LOOKS like a background, but is actually a "layer". That would mean your real background shows behind the "layer".
If this is what you discover, and their default code is unavoidable, then you would need to put your background image in a similar <div> (layer) with a higher z-index than theirs.
However, all that is just a guess. See what View Source tells you.