Relative CSS URL values with WebKit (Objective-C API)

I want to change the URL's of a background-image property in a HTML document.

DOMCSSStyleDeclaration *style = domELement.style;
NSString *imageURL = [style backgroundImage];

Now the imageURL looks something like url(<base URL I've loaded the HTML开发者_开发技巧 with>/image.jpg). But in the HTML source it's url(image.jpg). Now I'd like to change the image.jpg to an other image and still keep the relative path. How do I do that? Because when i set something like url(newImage.jpg) the rendered output looks like url(<base URL I've loaded the HTML with>/newImage.jpg).

May I change something when getting the HTML?

DOMHTMLElement *htmlElement = (DOMHTMLElement*)[document documentElement];
NSString *html = [htmlElement outerHTML];

Or is there an other way how I can set a relative URL in a css attribute?

[style setBackgroundImage:@"url(newImage.jpg)"];

I didn't found anything at all. So I did pars it my self with an NSSCanner