function preloadHero(images, callback) {
  var l = images.length;
  function loadImage(counter) {
    var img = new Image();
    // image loaded? call the next one
    img.onload = function() {
      counter++;
      // we done?
      if (counter == l) {
        if (callback && typeof(callback) === 'function') {
          callback();
        } else {
          return;
        }
      } else {
        // keep going
        loadImage(counter);
      }
    };
    // set the new image src to the image url
    img.src = images[counter];
  }
  // call the first one
  loadImage(0);
}