为什么焦点图下面会有黑框？

<script src="js/myfocus-2.0.1.min.js" type="text/javascript"></script>

<script src="js/mf-pattern/mF_YSlider.js" type="text/javascript"></script>

<link href="js/mf-pattern/mF_YSlider.css" rel="stylesheet" type="text/css"/>

<script type="text/javascript">

myFocus.set({

id:'picBox'}

)

</script>

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

<div class="loading">

<img src="images/loading.gif" alt="图片加载中">

</div>

<div class="pic"><!--需要命名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><!--ad结束-->