function newImage(arg) {
  if (document.images) {
    rslt = new Image();
    rslt.src = arg;
    return rslt;
  }
}

function changeImages() {
  document[changeImages.arguments[0]].src = changeImages.arguments[1];
}

var preloadFlag = false;
function preloadImages() {
  if (document.images) {
    index_on_17 = newImage("images/index_17.jpg");
    index_on_18 = newImage("images/index_18.jpg");
    index_on_19 = newImage("images/index_19.jpg");
    index_on_20 = newImage("images/index_20.jpg");
    preloadFlag = true;
  }
}