Or you could do this which I found on the interweb thingy.
https://robau.wordpress.com/2012/04/20/override-image-src-in-css/
<img src="linkToImage.jpg" class="egg">
.egg {
width: 100%;
height: 0;
padding: 0 0 200px 0;
background-image: url(linkToImage.jpg);
background-size: cover;
}
So effectively hiding the image and padding down the background. Oh what a hack but if you want an IMG tag with alt text and a background that can scale without using JavaScript?
In a project I'm working on now I created a hero block twig template
<div class="hero">
<img class="image" src="{{ bgImageSrc }}"
alt="{{ altText }}" style="background-image: url({{ bgImageSrc }});">
</div>