大小不固定圖片垂直居中

.pic_box {
    width: 300px;
    height: 300px;
    font-size: 0;
    text-align: center;
}
.pic_box:after {
    display: inline-block;
    width: 0;
    height: 100%;
    content: '.';
    vertical-align: center;
    overflow: hidden;
}
<div class="pic_box">
    <img scr=" "/>
</div>