[docbook-apps] Setting width for mediaobject in EPUB

[Xmplar]

I'm specifying the width of an image using the <imagedata> attribute 
'width' (and I've also tried 'contentwidth'). When I transform to an 
EPUB 3, I get the width coded as a style attribute:

<img style="width: 150; " src="images/by-nc.gif" alt="alt text for 
Creative Commons licence logo" />

The image doesn't render to a width of 150px. But removing the style 
setting and replacing it with the img attribute 'width' does work. Where 
do I find the stylesheet coding that sets style="width: ; ". I've looked 
in XHTML1-1/graphics.xsl and there's nothing obvious that sets the style 
attribute with a value of 'width' (all instance of 'style' refer to 
text-align).
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*Dave Gardiner*