When I played with some epub to kepub conversion, I found that to fit to screen, you need the svg wrapper. The <div> tag around the svg are there to make it compliant and keep FlightCrew and epubcheck from whimpering. When I converted a couple of books from epub to .kepub.epub, the cover display seems to look okay though I did not have a test case with an image larger than the Aura HD's display resolution.
Code:
<style type="text/css">
@page {padding: 0pt; margin:0pt}
body { text-align: center; padding:0pt; margin: 0pt; }
</style>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" height="100%" preserveAspectRatio="xMidYMid meet" version="1.1" viewBox="0 0 900 1200" width="100%" xmlns:xlink="http://www.w3.org/1999/xlink"><image height="1200" width="900" xlink:href="../Images/cover.jpeg"></image>
</svg>
</div>
Regards,
David