dynamic external images

I need to create external images on the fly and include the images in the 
document.

I try the following:

<xsl:variable name="image">
    <!-- the following template return an image file name -->
    <xsl:call-template name="create-image"/>
</xsl:variable>

<fo:external-graphic src="$image"/>

FOP does not allow variable used in the attribute.

Any workaround?

Thanks!

Larry