// images is an array of images name
function preloadImages(urlPrefix, images) {
  var imgPreload = new Array();
  var total = images.length;
  var now = 0;

  for (i = 0; i < images.length; i++) {

    imgPreload[i] = new Image();
    imgPreload[i].onload = function() {
      now++;
      if (now == total) {
        //alert("OK!");
        //end
      }
    }
    imgPreload[i].src = urlPrefix + images[i];
  };
}