为什么img的margin-top无效，margin-bottom有效？img已经设置为block。具体见下面的代码：
<style type="text/css">
#albumContainer{background-color:rgba(0,0,0,0.1);border-radius:10px;width:750px;margin:30px auto;}
#albumContainer img{display:block;width:25%;margin:10px auto;}
</type>
<li id="iLi2">
<div id="albumContainer">
<img alt="album" src="res/img/album001.png">
<ul id="albumList">
<li id="family">
<img alt="相册" src="res/img/photo/family/family0.jpg">
<span>family</span>
</li>
<li id="sister">
<img alt="相册" src="res/img/photo/sister/sister0.jpg">
<span>dear sister</span>
</li>
<li id="cat">
<img alt="相册" src="res/img/photo/cat/cat0.jpg">
<span>big lazy cat</span>
</li>
<li id="me">
<img alt="相册" src="res/img/photo/me/me000.png">
<span>me</span>
</li>
</ul>
</div>
</li>
显示的结果为:
为什么在上面对img使用了margin:10px auto;对下外边距有效，对上外边距却没有效果呢？
