Responsive images adapt to the screen and its pixel density: srcset offers several files, sizes indicates the displayed width so the browser can pick the most suitable one, and <picture> lets you switch format or cropping. They reduce the downloaded weight.
Example
<img src="p-800.jpg" srcset="p-400.jpg 400w, p-800.jpg 800w" sizes="(min-width: 48rem) 50vw, 100vw" alt="">