<div class="img_box">
<img src="1.jpg" alt="" class="img_newItem">
</div>
父级设置宽高和flex布局，子级图片设置最大宽高（max-width\max-height）为100%，宽高(width\height)为auto；
.img_box{
width:100%;
height:100%;
display: flex;
justify-content: center;
align-items: center;
}
.img_newItem{
max-width: 100%;
max-height: 100%;
width: auto;
height: auto;
}
