This websoyte goes a step further: lazy loading via Javascript.
All images are already loaded, but will display as white boxes if you have JS disabled, so in order to see you'll need to change "display: none !important;" to "display: block !important;" for the "lazyload" CSS class, and manually delete "decoding="async" src="data:image/TROLOLOLO"" attributes, and change the "data-src" attribute to "src" on each image.