<?php
$vet_image = array("img1.jpg","img2.jpg","img3.jpg");
$image = array_rand($vet_image);
?>

<html>
<head>
<style type="text/css">
<!--
body {
background-image: url(images/<?php print $image; ?>);
}
-->
</style></head>

<body>
</body>
</html>