addEventSimple(window, 'load', function() {
  var main_image_link = $('main_image_link');
  var main_image = main_image_link.getElementsByTagName('img')[0];
  
  var product_thumb_links = $('product_thumbs').getElementsByTagName('a');

  // change main image link to a popup window  
  main_image_link.onclick = function() {
    var newwindow = window.open(this.href, 'Details', 'left=50,top=50,width=800,height=600');
   	if (window.focus) newwindow.focus();
    return false;
  }
  
  // add navigation behaviour to the thumbnail links
  var i, thumb;
  for (i = 0; thumb = product_thumb_links[i]; i++) {
    thumb.image = thumb.getElementsByTagName('img')[0];
    thumb.onclick = function() {    
      main_image.src = this.image.src;
      main_image_link.href = this.href;
      return false;  
    }
  }

});