通过在图片后面加一个隐藏span间接实现img居中，
代码如下：
<style type="text/css">
div{ border:1px solid #ccc; height:500px; 500px; text-align:center; margin:0 auto; background:#ccc;}
img{ vertical-align:middle; }
div span{ height:100%; 0; overflow:hidden; display:inline-block; vertical-align:middle; }
</style>
<div class="test">
<img src="icon.png"><span> </span>
</div>
还有一种方式，借助table布局：
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head>
<meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
<title>无标题文档</title>
<style type="text/css">
*{ margin:0; padding:0;}
#box{
500px;height:400px;
text-align:center;
border:1px solid #d3d3d3;background:#fff;
display: table;
*display:block;
*font-size:349px; // 字体大小约为容器高度的0.873倍 400*0.873 = 349
*font-family:Arial; // 防止非utf-8引起的hack失效问题，如gbk编码
}
#box span{
display: table-cell; vertical-align:middle;
}
#box img{
*vertical-align:middle;
}
</style>
</head>
<body>
<div id="box">
<span><img src="http://www.w3cfuns.com/resource/images/extend/logo/w3cfuns_logo_hd.png" width="420" height="300" alt="" /></span>
</div>
</body>
</html>