js中网页图片自动更换的效果

<script>
 var arr=new Array();
 
 arr[0]="url(images/city.jpg)";
 arr[1]="url(images/desert.jpg)";
 arr[2]="url(images/flower.jpg)";
  var i=0;
  function  changeimage()
  {
      if(i==3)
      {
        i=0; 
      }
    var div=document.getElementById("apdiv1");
    div.style.backgroundImage=arr[i];
    i++;
    window.setTimeout("changeimage()",1000);
  }
  changeimage();
  
  
 </script>