if (document.images){
  preload_image_object = new Image();
  image_url = new Array();
  image_url[0] = "images/btn_01_on.gif";
  image_url[1] = "images/btn_02_on.gif";
  image_url[2] = "images/btn_03_on.gif";
  image_url[3] = "images/btn_04_on.gif";
  
  var i = 0;
  for(i=0; i<=3; i++) 
   preload_image_object.src = image_url[i];
 }