Relative path names

Hi,

In one of my customization layer stylesheets, I have the following excerpt:
  <fo:block text-align="center">
    <fo:external-graphic src="url(/path/to/some/image.png)"
content-type="content-type:image/png" text-align="center"/>
  </fo:block>

Is there any way of making that path relative to the stylesheet?

thanks,
Remko