<!DOCTYPE html>
<html>
<head>
<meta charset=utf-8" />
<title>jQuery实现页面背景随机切换</title>
<style>
.back {
  position: absolute;
  width: 1000px;
  height: 100px;
  background-size: cover;
}
</style> 
<script src="http://libs.baidu.com/jquery/1.10.2/jquery.min.js"></script>
<script>
$(document).ready(function(){
  var imgArr=["bg001.jpg", 
        "bg002.png", 
        "bg003.png"
  ];
  var img =parseInt(Math.random()*(imgArr.length)); 
  var currentImage=imgArr[img];
  $(".back").css("background-image","url("+currentImage+")");
});
</script>

</head>
<body>
  <div class="back" > 
    123
  </div>
</body>
</html>