var imgArr=["http://www.google.com.hk/intl/zh-CN/images/logo_cn.png",
"http://www.baidu.com/img/baidu_sylogo1.gif",
"http://www.open-open.com/news/uploadImg/20120111/20120111081906_79.jpg",
"http://www.open-open.com/news/uploadImg/20120111/20120111081906_76.jpg"
];

var index =parseInt(Math.random()*(imgArr.length-1));

var currentImage=imgArr[index];

document.getElementById("BackgroundArea").style.backgroundImage="url("+currentImage+")";

<div id="BackgroundArea">
</div>