activated object

this represents the activated object
ex. this.id , this.className += "highlight"

if (cellIsEmpty(this)) {
    // User clicked on an empty cell
    alert("Please click on a numbered tile.");
    return;
}

scroll images

function scrollImages() {
  var coverBarDiv = document.getElementById("coverBar");
  var images = coverBarDiv.getElementsByTagName("img");
  for (var i = 0; i < images.length; i++) {
    var left = images[i].style.left.substr(0, images[i].style.left.length - 2);
    if (left <=  -86) {
      left = 532;
    }
    images[i].style.left = (left - 1) + "px";
  }
}