[xsl] External-graphic

Hi,
I am new to xsl and want to display an image in a pdf file. I am using
the following syntax in the stylesheet.

<fo:block text-align="center">
	<fo:external-graphic src="file:c/logo.jpg"  height="3cm"
width="3cm"/>
</fo:block>

Nothing happens. What could be the problem?
Thanks

Tom


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