open you image in an image editor and make the dimension 600x800px
in the cover.xhtml file
change the code to match this:
PHP Code:
<?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>BookName</title>
</head>
<body id="toc_marker-1" style="margin-top:0px; margin-left:0px; margin-right:0px; margin-bottom:0px; text-align:center;" >
<div><img src="image/Cover.jpeg" alt="Cover.jpg" height="100%" />
</div>
</body>
</html>