DOCBOOK: Fop-0.20.1 Image Problems

I just tried to render PDFs from docbook using the xsl stylesheets from 
Norman Walsh with the most recent Apache fop Fop-0.20.1 version.

there seem to be yet lots of problems with it. but one is strange: using 
xep, the created PDF is o.k. but using fop, the images (gifs) are not 
included with the error message, that the file could not be found.

now I looked into the generated fo file, there can be found:

<fo:external-graphic height="auto" width="auto" content-height="auto" 
content-width="auto" src="url(dilbert_747.gif)"/>

this works with xep, not with fop, when I change this to:

<fo:external-graphic height="auto" width="auto" content-height="auto" 
content-width="auto" src="dilbert_747.gif"/>


it works with fop. where is the problem? is this a fop problem?


btw. I am getting mad with all of these fo problems... there seem to be 
no implementation that allows a stable transformation to PDF, except the 
extremly expensive xep.



thanks


Alex