SOURCE

<div class="box">
  <img src="http://static.jsbin.com/images/dave.min.svg" alt=''>
  <!-- <img src="http://static.jsbin.com/images/dave.min.svg" alt=''>
  <img src="http://static.jsbin.com/images/dave.min.svg" alt=''> -->
</div>

.box {
  border:1px solid #aaa;
  width:400px;
  height: 400px;
  display: flex;
  justify-content: space-around;
  background-color: green;
 /*  主轴对齐方式 水平居中 */
  align-items: center;
 /*  交叉轴对齐方式 垂直居中*/
}
img {
  height: 100px;
}