The quick answer is no.

It is a intrinsic problem with fluid layouts in general within html /css, not just Zen Cart. The background image is not scaled with it's container. It makes sense really, the image is displayed at the right resolution. If you start re-sizing images then the resolution goes to pot and they look awful anyway.

The problem is that most elements of a page can be made fluid. Images really can't. There are some technical solutions out there but none of them are a quick solution.

There are various workarounds that make the problem a bit less painful. If you post a URL people can have a look and perhaps suggest some ideas....