Lots of different techniques to properly set up images for responsive & retina screens.

<picture>
<!-- Put the biggest at the top, because the first appropriate one will be selected -->
<source media="(min-width: 60em)" srcset="large.jpg">
<source media="(min-width: 38em)" srcset="medium.jpg">
<img src="small.jpg" alt="A giant squid swimming deep in the sea">
</picture>