How to use an image path passed in an XML file to use

I would like to know how the complete url passed as a data in xml with
tag staffPhoto ,which can be used in <fo:external-graphic src=.....> ?

I tried to read the xml line which is generated via a program in this way,

<staffPhoto> url('file:C:///staff/beena.jpg') </staffPhoto>

In my xsl i wrote the below line to read the above xml line

<fo:external-graphic src="staffPhoto"/>

What is wrong in this ? I got the error message, Error with image URL
:staffPhoto (the system cannot find the file specified) and no base
URL is specified.

But if i copy paste the url path , the image will be loaded without any problem.

Please help me to solve this problem  and is very crucial in my project.

I also tried the way you suggested to use the xsl:param ...

I am using 
xml version 1.0 encoding "ISO-8859-1".
xsl:stylesheet version 1.0
xmlns:xsl="http://www.w3.org/1999/XSL/Transform"
xmlns:fo="http://www.w3.org/1999/XSL/Format"

Thanks a lot.
Beena