网站导航二级菜单显示出来但被图片遮住了；

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<html xmlns="http://www.w3.org/1999/xhtml">function show(n){
document.getElementById(n).style.display="block";
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<div id="about_nav" onmouseover="show('about_nav')" onmouseout="hide('about_nav')">
         <img src="images/index/nav/nav_2_top.png" />
         <a class="about_nav" href="about.html" target="_parent">公司简介</a>
            <a class="about_nav" href="about_qywh.html" target="_parent">企业文化</a>
            <a class="about_nav" href="about_jzys.html" target="_parent">竞争优势</a>
            <a class="about_nav" href="about_history.html" target="_parent">成长历程</a>
            <a class="about_nav" href="about_zs.html" target="_parent">公司资证</a>
            <a class="about_nav" href="about_wl.html" target="_parent">公司网络</a>
            <a class="about_nav" href="about_gsjg.html" target="_parent">公司结构</a>
            <a class="about_nav" href="about_news.html" target="_parent">最新动态</a>
            <img src="images/index/nav/nav_2_bottom.png" />
        </div>