Untitled

<html>
<head>


<script language="JavaScript">
                 winW = window.innerWidth;
                 winH = window.innerHeight;

                document.html.style.backgroundImage = "url(path/to/image)"
</script>

<title>MG</title>


<style type="text/css">
html {
        -webkit-background-size: cover;
        -moz-background-size: cover;
        -o-background-size: cover;
        background-size: contain;
}
</style>
</head>
<body>
</body>
</html>