Fw: fo:external-graphic

Hi @All,

I' m trying to load an image into my PDF Document, using this statement:

 <xsl:variable name="servicecontract"><xsl:value-of select="//session/projectName"/>/img/servicecontract.gif</xsl:variable>

       <fo:external-graphic
         src="{$servicecontract}" content-height="100%" content-width="100%"/>

But the image doesn't appear in the Pdf.

I' wondering because i've tried with an 
       <img src="{$servicecontract}" border="0" alt=""/>

which works fine on an html output from the same sheet.

Pleas can anybody help me ?!

Thanxs