DOCBOOK: Problems with imagedata

Hi,
In a docbook document I wrote something like this:

<mediaobject>
  <imageobject>
    <imagedata fileref="images/sd-expreg.png" format="PNG"/>
  </imageobject>
  <textobject>
    <phrase>Sequence Diagram "Export Registrations"</phrase>
  </textobject>
  <caption>
    <para>
      This diagram depicts object interactions arranged in time
      sequence for the "Export Registrations" scenario.
    </para>
  </caption>
</mediaobject>

Well, when I generate (with jade) HTML documents, everything
works fine, but when I generate postscript files, they
do not include the image specified by imagedata.

Are there any issue with images (image files are stored in the
images subdirectory)?

Thanks

Giuseppe