jsでランダムに切り替わる背景画像を設定する方法

<script>
   
var imgs =["http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_2.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_default.png",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_3.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_1.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_4.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_5.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_6.jpg",
       "http://images.cnblogs.com/cnblogs_com/xiaobo-Linux/1112236/o_default-2.jpg",
       ];  
   
var index=Math.floor(Math.random()*8);
var img = imgs[index];
    function time(){
          
       document.body.style.backgroundImage="url("+img+")";
          
    }  
   
document.body.onload = function(){
 time();
}
</script>