Dynamic Images in FO

I would like to display a dynamic image in an FO stylesheet.
 
With it hard coded I have:
 
<fo:external-graphic src="c:\DPS PO Images\5.gif"/>
 
What I need is to replace the 5 with an <xsl:value-of
select="DynamicNumber"/> but it does not allow you to put XSL within the
FO.
 
How can I achieve this?
 
Thanks