<style>
.image-header{
position: relative;
width: 100%;
height: 0;
padding-top: 100%;
}
.image-header img{
position: absolute;
top: 0;
left: 0;
width: 100%;
height: 100%;
}
</style>
<div class="image-header">
<img src="demo.jpg">
</div>
最終圖檔以正方形顯示