window.onload = function() {
  var path = 'images/'
  var img = ['cruiser_beige.gif', 'cruiser_red.gif', 'cruiser_blue.gif', 'cruiser_yellow.gif', 'cruiser_red_truck.gif'];
  document.images.rand.src = path + img[Math.floor(Math.random() * img.length)];
}