If you want it width-resizable, then you can split the background into two parts and use the
Sliding Doors technique.
However this requires adding markup to your elements, because you're essentially layering two backgrounds. CSS3 allows multuple backgrounds per element, but that's still out of reach.
If you want the elements to be both width- and height-resizable, then it's going to involve more CSS wizardry and a lot more markup. For instance, to achieve rounded corners, there's about a dozen of different methods, and each one uses complex tangles of CSS and tons of extra HTML markup. And that's just for the corners: if you want to make an entire frame out of images, then it's even messier. Here, too, CSS3 will eventually save the day, with support for border images as well as built-in rounded corners.