Our WSRP implementation doesn't currently handle resources very well. background:url in style elements are indeed not currently properly detected as targets for rewriting.
A possible solution would be a store the static resources such as images in an easily reachable location and refer to them via an absolute URL, though I admit that this is sub-optimal.
Thanks for your response.
It's a bad news for us. All our resources are given by dynamic URL (we use JSF).
We will consider a new solution for our resources.