js如何设置随机切换背景图片

<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",
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 ]; 
 
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>