function preloadImages(){
  if(document.images){
    var imgFiles = preloadImages.arguments;
    var preloadArray = new Array();
   for(var i=0; i < imgFiles.length; i++){
     preloadArray[i] = new Image;
     preloadArray[i].src = imgFiles[i];
   }
  }
 }

preloadImages('{$host_name}pictures/button_active_over.gif', '{$host_name}pictures/button_active.gif', '{$host_name}pictures/button.gif', '{$host_name}pictures/button_over.gif', '{$host_name}pictures/sub_button_over.gif','{$host_name}pictures/sub_button.jpg');