Apache Fop: Galssfish 3.1.2 fails to render pictures

Question

I'm trying to add a picture to my pdf-file withe the fo:external-graphic-tag:

<fo:external-graphic src="url(resources\common\img\Logo_tiny.png)" />

On my local System everyting works fine, but when i want to run it on the Server-System it says

Image not found. URI: resources\common\img\Logo_tiny.png. (No context info available)

My Fop-Factory-Setup looks like this:

    fopFactory.setUserConfig(config);
    fopFactory.getFontManager().setFontBaseURL(servletContext.getRealPath("/WEB-INF/config/"));
    Fop fop = fopFactory.newFop(org.apache.xmlgraphics.util.MimeConstants.MIME_PDF, userAgent, outStream);
    Transformer transformer = TransformerFactory.newInstance().newTransformer(transformSource);

    Result res = new SAXResult(fop.getDefaultHandler());

    transformer.transform(source, res);

    // get the pdf:
    response.setContentType("application/pdf");
    response.setHeader("Content-Disposition", "attachment; filename=\"Output.pdf\"");
    response.setContentLength(outStream.size());

    OutputStream responseOutputStream = response.getOutputStream();
    outStream.writeTo(responseOutputStream);

    responseOutputStream.flush();
    responseOutputStream.close();
    context.responseComplete();

I checked everything a couple of times: the image is available in the jar-file it is available in the ear-file. The picture is also used in a different context nd there it shows up.

Does anyone have a hint or a solution for this problem? Many Thanks!!