css图片宽高相等设置

<div class="image-header">
<img src="demo.jpg">
</div>

.image-header{
position:relative
width:100%
height:0
padding-top:100%

}
.image-header img{

position:absolute
top:0
left:0
width:100%
height:100%

}

最终图片以正方形显示