OPF file for EPUB is linking to images with different roles

For Docbook 4.x, I was able to use the following to force PDF's to use 
SVG and HTML Books to use PNG's:

<mediaobject>
<imageobject role="fo">
<imagedata fileref="screenshots/wizard_step_1.svg" scale="70"/>
</imageobject>
<imageobject role="html">
<imagedata fileref="screenshots/wizard_step_1.png" scale="70"/>
</imageobject>
</mediaobject>

This still works for PDF and HTML documents, however, I am finding that 
the epub translation causes both of these images to be referenced in the 
OPF file:

<manifest>
...
<item id="d0e88" href="screenshots/wizard_step_1.svg" 
media-type="image/svg+xml"/>
<item id="d0e94" href="screenshots/wizard_step_1.png" 
media-type="image/png"/>
...
</manifest>

The raw html output of the translated epub is (correctly) only 
referencing the .png file

<div class="mediaobject"><img src="screenshots/wizard_step_1.png" 
width="378" alt="Step 1 Image Data"/>

Incidentally, what "role" should an epub document be? role="epub" does 
not seem to work. Is it "xhtml"?

Glenn