You can simply browse to a specific file location, or specify a URI as the source attribute of an image.
Dynamic location of file based on xml values is simply acheived using xslt attribute templates. Consider the following
<fo:external-image src="http://imageserver/get?{product_id}"/ >
xsl-fo
The example above demonstrates the use of attribute templates to retrieve image from a server.
Note portion wrapped in curly brackets { }.
As the image object wraps the xsl-fo implementation, you can alter the src property field as follows:
http://imageservlet/get?{product_id}
Make sure you enter reserved xml characters with their entity reference
Example & -> &
Image source url with parameters to output a QR-code image