#btn {    width:40px;    height:40px;    position:fixed;    left:50%;     margin-left:610px;    bottom:30px;     background:url(http://img.mukewang.com/535e0dc100010e9c00400080.jpg) no-repeat left top;    }#btn:hover {    background:url(http://img.mukewang.com/535e0dc100010e9c00400080.jpg) no-repeat 0 -39px;    }.box {    width:1190px;     margin:0 auto;    }

<div class="box">        <img src="http://img.mukewang.com/535e0ce800015b7511902787.jpg" />    </div>    <a href="javascript:;" id="btn" title="回到顶部"></a>