function setRandomBackground()
{
   // random() * 4 yields number between 0 and 3
   class_num = (Math.floor(Math.random() * 4)) + 1

   // set class name
   div = document.getElementById('banner');
   
   if (div.className) {
      div.className += ' banner' + class_num;
   } else {
      div.className = 'banner' + class_num;
   }
}