Foren / Cotonti / Skins / random logo css

Easiest solution is to use JavaScript:

<script type="text/javascript">
$(function() {
  var logos = ["../img/logo1.png", "../img/logo2.png", "../img/logo3.png"];
  $('#header').css('background-image', 'url(' + logos[Math.floor(Math.random()*logos.length)] + ')');
});
</script>

Put it in header.tpl, right before </head>