<script type="text/javascript">  
  window.onload = changeBG;  
  function changeBG() {  
  var first = 1;  
  var last = 4;  
  var path = '/imgfon/';  
  var img_src = 'url("' + path + getRandomInt(first,last) + '.jpg")';  
  var div = document.getElementById("main");  
  div.style.backgroundImage=img_src;  
  }  
  function getRandomInt(min, max)  
  {  
  return Math.floor(Math.random() * (max - min + 1)) + min;  
  }  
  </script>