js中网页图片自动更换的效果

<script>
 var arr=new Array();

 arr[]="url(images/city.jpg)";
 arr[]="url(images/desert.jpg)";
 arr[]="url(images/flower.jpg)";
  var i=;
  function  changeimage()
  {
      if(i==)
      {
        i=;
      }
    var div=document.getElementById("apdiv1");
    div.style.backgroundImage=arr[i];
    i++;
    window.setTimeout("changeimage()",);
  }
  changeimage();

 </script>