> This assumes, that all CSS image URLs are relative to the base URL. That may not be always the case.

Please can you give an example of when that is not the case?

We face a problem here. Because it's also true that we can't always locate the CSS image from knowing the CSS file path. For example the website might be using `mod_rewrite` rules so the url has no bearing on the actual file on disk.

I guess, we can try a bunch of likely locations and also allow to specify some folders, in the config. But that's getting ugly. I'll test some things before updating this pull-request.