xsl can't get graphic to load from xml file

I am trying to get a graphic who's path is stored in
my xml file.  I can hard code the graphic path in my
xsl file and this bit of code works but hard coding is
notever that way to go. Can someone give me a
statement that will display the graphic that is not a
hardcoded url? I want to get the string returned from
the template logo and use it for the url.  Ideas? 
This is what I have.

<xsl:template name="logo">
	<xsl:value-of select="invoice/images/logo"/>
</xsl:template>

<xsl:template name="getLogo">
	<fo:external-graphic src="url( D:\simple\logo.jpg)"/>
</xsl:template>


Thanks!



		
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