一张矩形的图片，如何得到圆形的头像效果？

<style>
img {
    border: 3px solid #cccccc;
    border-radius: 50%;
    height: 80px;
    width: 80px;
}
</style>
<img src=http://www.mamicode.com/"http://mt1.baidu.com/timg?wh_rate=0&wapiknow&quality=100&size=w250&sec=0&di=357d1834400b498793f7306937bd9035&src=http%3A%2F%2Fimg.iknow.bdimg.com%2Fjctuijian%2F0819%2F4.jpg">