SOURCE

<div class="box1">
    <!-- 横图 -->
    <img src="https://ss1.bdstatic.com/70cFuXSh_Q1YnxGkpoWK1HF6hhy/it/u=3756113378,2176758247&fm=26&gp=0.jpg">
</div>
<div class="box2">
    <!-- 竖图 -->
    <img src="https://ss3.bdstatic.com/70cFv8Sh_Q1YnxGkpoWK1HF6hhy/it/u=4126463820,1175275897&fm=26&gp=0.jpg">
</div>

img {
    width: 200px;
}

.box1 {
    position: relative;
    z-index: auto;
}

.box1 img {
    position: absolute;
    z-index: 2;
}

.box2 {
    position: relative;
    z-index: auto;
}

.box2 img {
    position: relative;
    z-index: 1;
}