this was actually harder to find than I thought it would be (I needed the same ability).
var xBG = xStyle.backgroundImage;
this should return the full path in the CSS definition, including "url(...)". I typically define background-image:url(...);, background-position:...; background-repeat:...;
In short, I don't know what happens if you use short hand like background: url(...) left top #000;, for example.
Hope this helps you!