imagedata scale problem

Hi List.
I have some troubles when rendering images specified as

<equation>
      <title>Mitochondrial Membrane Potential</title>
	<alt/>
	<mediaobject>
		<imageobject>
             	<imagedata role="analysis" fileref="/images/xml/53_2.png" scale="80" format="PNG"/>
		</imageobject>
      </mediaobject>
</equation>

When using the HTML DB5 stylesheet, the result for the image is

<div class="equation-contents">
      <div class="mediaobject">
		<img width="432" src="/images/xml/53_2.png">
	</div>
</div>

My images all have the same resolution, thus different size but uniform display appearance of the equation they contains. As they are a bit too big, I use scale=80" to reduce their size by 20%. My problem is that the result gives for all images an <img width="432"...>  whatever the intrinsic image size is. Thus my equations appears now all with different display sizes on the HTML page, which is not the expected output.
As a hint, when using xsl-fo output, I get
<fo:block id="d68e38942">
<fo:external-graphic src="url(/images/xml/53_2.png)" width="auto" height="auto" content-width="80%" content-height="80%" content-type="content-type:image/png"/>
</fo:block>
Which is correct.

Any ideas ?
Regards,
Fabien