图片特效盖住菜单的时候，修改position，和zIndex就可以了(display:block的元素同样适用)

function displaySubMenu(li) {

            var subMenu = li.getElementsByTagName("ul")[0];
            subMenu.style.position = "absolute";
            subMenu.style.zIndex = "20";
            subMenu.style.display = "block";

        }