Hi ahmadlubiss,
You must have define an array of images.
Then on window.onload event handler set the random image to the backgroundImage property of the body element.
HTML
<html>
<head>
<meta charset="utf-8" />
<title></title>
<script type="text/javascript">
window.onload = function () {
var images = ['Images/1.jpg', 'Images/2.jpg', 'Images/3.jpg', 'Images/4.jpg', 'Images/5.jpg', 'Images/6.jpg', 'Images/7.jpg'];
var image = images[Math.floor(Math.random() * images.length)];
document.getElementsByTagName('body')[0].style.backgroundImage = "url('" + image + "')";
document.getElementsByTagName('body')[0].style.backgroundRepeat = "no-repeat";
document.getElementsByTagName('body')[0].style.backgroundSize = "cover";
}
</script>
</head>
<body>
</body>
</html>
Screenshot
