if (document.images) {
  image1on = new Image();
  image1on.src = "images/trolley100_on_ani.gif";

  image1off = new Image();
  image1off.src = "images/trolley100_ani.gif";
}
function changeImages() {
  if (document.images) {
    for (var i=0; i<changeImages.arguments.length; i+=2) {
      document[changeImages.arguments[i]].src = eval(changeImages.arguments[i+1] + ".src");
    }
  }
}