DOCBOOK-APPS: Transforming DocBook to PDF with FOP

I use FOP to transform a DocBook XML file. My source file contains a lot of 
screen shot.

Step 1: I transform XML DocBook file to a FO file with Norman XSL. 
References to image file look like :

<fo:external-graphic height="auto" width="auto" content-height="auto" 
content-width="auto" src="url(file:img/resultat_image.jpg)"/>

Step 2: I use FOP to transform my FO file to a PDF file. And I get an error 
message for each image file reference:

Error while creating area : no protocol : url(file:/img/resultat.jpg)

Does anyone have a idea of what occurs? I've tried to give an absolute path 
to images files without success.

Thanks by advance.

Fr嶮廨ic Demians