Replace your code with this...
Spoiler:
PHP Code:
<?xml version='1.0' encoding='utf-8'?>
<html xmlns="http://www.w3.org/1999/xhtml" xmlns:epub="http://www.idpf.org/2007/ops" xmlns:xlink="http://www.w3.org/1999/xlink">
<head>
<meta name="calibre:cover" content="true"/>
<title>Cover</title>
<style type="text/css" title="override_css">
@page {
padding: 0;
margin: 0;
}
body {
text-align: center;
padding: 0;
margin: 0;
}
</style>
</head>
<body>
<div>
<svg xmlns="http://www.w3.org/2000/svg" version="1.1" width="100%" height="100%" viewBox="0 0 1600 2560" preserveAspectRatio="xMidYMid meet">
<image width="1600" height="2560" xlink:href="../OEBPS/cover.jpg"/>
</svg>
</div>
</body>
</html>
You will need to adjust the viewbox and the width and height values to match your cover image dimensions and relink the href: image.
Remember to make a backup first, so you can easily revert if needed.