Hiya,
the img element doesn't support that syntax. Use the chapter@pages attribute instead, e.g. something like this:
<?xml version="1.0" encoding="utf-8"?>
<!DOCTYPE book SYSTEM "http://megazine.mightypirates.de/megazine.dtd">
<book bgcolor="0x000000" pageheight="432" pagewidth="340" lang="en" barpos="20" reflection="true" bggradient="false">
<chapter>
<page stiff="true">
<img src="../imgs/normal/cover.jpg" hires="../imgs/hires/cover.jpg" />
</page>
</chapter>
<chapter pages="../imgs/normal/page[2-162].jpg|_hires">
</chapter>
<chapter>
<page stiff="true">
<img src="../imgs/normal/back.jpg" hires="../imgs/hires/back.jpg" />
</page>
</chapter>
</book>
The hires images will have to be in the same folder as the normal ones, though, and have the suffix
_hires (in the above example).