[xsl] insert an image into a pdf

Hello,

I run an xsl stylesheet to make a pdf which contains the expression:
<fo:external-graphic src='{$logo}' ...

I use this expression because I want to insert an icon.

It works fine when $logo=file:c:/icons/logo.jpg, but I would like to
read this image from a jar file and in this case I have not found the
good syntax.
I have tried $logo=file:archive.jar/icons/logog.jpg but still
unsuccessful.

Is it possible to do it by this way ?

Thanks for your help

Steph