I cannot find it (see below)
Also in the Kobo Touch plugin I have 'keep cover aspect ratio' checked.
<?xml version="1.0" encoding="utf-8" standalone="no"?>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.1//EN"
"http://www.w3.org/TR/xhtml11/DTD/xhtml11.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<title>Cover</title>
<link href="../Styles/template.css" rel="stylesheet" type="text/css" />
<style type="text/css">
@page {padding:0; margin:0;}
img{height: 100%;}
</style>
</head>
<body style="padding:0; margin:0;">
<div class="image" id="my_cover_image"><img alt="Cover" height="764" src="../OEBPS/cover.jpg" width="550" /></div>
</body>
</html>