I just use Sigil's add-a-cover tool, which creates cover.xhtml and something like this:
<div style="text-align: center; padding: 0pt; margin: 0pt;">
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 1410 2000" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="2000" width="1410" xlink:href="../Images/CoverName.jpg"></image></svg>
</div>
I think it's the width="100%" that's key.
AFAIK it works everywhere, though I seem to recall that the online Nook preview clips it (showing a quadrant rather than the whole thing), though the downloaded Nook epub works fine.
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