/*css代码*/

.ad{

     height: 320px;

     overflow: hidden;

}

<!--html代码-->

<div class="ad" id="picBOX">

     <div class="pic">

         <ul>

             <li><img src="images/ad2.jpg"></li>

             <li><img src="images/ad3.jpg"></li>

             <li><img src="images/ad4.jpg"></li>

         </ul>

     </div>

</div>