try this:

<external-graphic entity-system-id="foo.gif"/>

ht
-- 
  Henry S. Thompson, HCRC Language Technology Group, University of Edinburgh
     2 Buccleuch Place, Edinburgh EH8 9LW, SCOTLAND -- (44) 131 650-4440
	    Fax: (44) 131 650-4587, e-mail: ht@xxxxxxxxxxxxxxx
		     URL: http://www.ltg.ed.ac.uk/~ht/


 XSL-List info and archive:  http://www.mulberrytech.com/xsl/xsl-list