window.addEvent("domready",function() {
  var duration = 4500;
  var image_width = 500;
  var page_size = document.id('wrapper').getSize();
  var start_pos = page_size.x;
  var end_pos = 0 - image_width;

  var animator;
  var begin_animation = function() {
    animator.tween("background-position", [start_pos + "px 0px", end_pos + "px 0px"]);
  };

  animator = document.id('wrapper').setStyle("background-position", page_size.x + "px 0px").set("tween", {
    duration: duration,
    transition: Fx.Transitions.linear,
    onComplete: begin_animation
  });

  begin_animation();
});