Try wrapping your cover image in a svg wrapper. The ModifyEpub plugin for calibre uses the following code:
Code:
<style type="text/css" title="override_css">
@page {padding: 0pt; margin:0pt}
body { text-align: center; padding:0pt; margin: 0pt; }
</style>
</head>
<body>
<div>
<svg version="1.1" xmlns="http://www.w3.org/2000/svg"
xmlns:xlink="http://www.w3.org/1999/xlink"
width="100%" height="100%" viewBox="0 0 1125 1500"
preserveAspectRatio="xMidYMid meet">
<image width="1125" height="1500" xlink:href="images/cover.jpeg"/>
</svg>
</div>
Please note that the 1125 x 1500 are the dimensions of the cover image and should match the cover image dimensions.