if (document.images)
{
  preload_image_object = new Image();
  // set image url
  image_url = new Array();
  image_url[0] = "images/layout/bg-intro-block.gif";
  image_url[1] = "images/layout/bg-line.gif";
  image_url[2] = "images/layout/enquiry-btn.gif";
  image_url[3] = "images/layout/enquiry-btn-hover.gif";

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