Add "lazy" CSS class to any image, iframe.
Use data-src attribute for images and iframes:
<img class="lazy" src="" data-src="https://picsum.photos/id/1001/1200/480/">
Use data-src-background attribute for background images:
<div class="lazy thmb bg-c" data-src-background="https://picsum.photos/id/1051/1200/480/">
You can even use it to lazy load iframe embeds.