function setImage(index) {
  // clear all
  for (i=0; i<7; i++) {
    document.getElementById('dot'+i).setAttribute('class', 'none');
  }
  // set it to current
  document.getElementById('dot'+index).setAttribute('class', 'current');
  // load image
  img = new Image();
  img.src = 'screen'+(index+1)+'.png';
  document['screenshot'].src = img.src;
}