.demo li{
   	    position: relative;
   	    float: left;
   	}
   	.demo img{
   	    display: none;
   	    position: absolute;
   	    top: 150%;
   	    left: 50%;
   	    margin-left: -60px; 
   	    /*这个margin-left的值是图片宽度的一半*/
   	}
   	.demo li:hover img{
   	    display: block;
   	}