PHP код:
<?
$bg = array();
$bg[]="image1.gif";
$bg[]="image-another-name.gif";
$bg[]="fun-picture-with-asshole.jpg";
$bg[]="britney-spears.png"
$count = count($bg);
$show = mt_rand(0,$count);
echo "<style type='text/css'>\nbody {\nbackground-image: url(".$bg[$show].");\n}\n</style>\n";
?>