Edit in JSFiddle

<div>
  <div style="position:relative; z-index:auto;">
    <img src="https://demo.cssworld.cn/images/common/l/1.jpg" style="position:absolute; z-index:2;">
  </div>
  <div style="position:relative; z-index:auto;">
    <img src="https://demo.cssworld.cn/images/common/l/2.jpg" style="position:relative; z-index:1;">
  </div>
</div>
<div>
  <div style="position:relative; z-index:0;">
    <img src="https://demo.cssworld.cn/images/common/l/1.jpg" style="position:absolute; z-index:2;">
  </div>
  <div style="position:relative; z-index:0;">
    <img src="https://demo.cssworld.cn/images/common/l/2.jpg" style="position:relative; z-index:1;">
  </div>
</div>

img {
  width: 100px;
}