<div> instead of <p> will get rid of any built-in paragraph styling. The height setting may not agree with Sigil Preview, but I think a reader will cope.
Have you set the semantics for both cover page and cover image?
<?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>Omslag</title>
</head>
<body style="margin: 0px; padding: 0px;">
<div style="text-align: center;"><img alt="omslag7f" src="../Images/omslag7f.jpg" height="100%" /></div>
</body>
</html>