function rollover(imgID) {
  // get the image object we're referring to
  var thisimg = document.getElementById(imgID);
  // and add "_on" to its src
  thisimg.src = thisimg.src.replace(/(\.[a-z0-9]+)$/i,'_on$1');
}
function rollout(imgID) {
  // get the image object we're referring to
  var thisimg = document.getElementById(imgID);
  // and remove "_on" from its src
  thisimg.src = thisimg.src.replace(/_on(\.[a-z0-9]+)$/i,'$1');
}

preload_image_object = new Image();
      // set image url
      image_url = new Array();
      image_url[0] = "images/roll1_on.jpg";
      

       var i = 0;
       for(i=0; i<=3; i++) 
	   preload_image_object.src = image_url[i];