You can check out the getkirby.com source code on GitHub: https://github.com/getkirby/getkirby.com
As far as I can see, there is an imgix plugin in the /site/plugins folder. Then images are called like this:
<img src="<?php echo imgix($image, array('w' => 300)) ?>" alt="Screenshot: <?php echo $feature->title() ?>" />
Maybe this new plugin is interesting for you as well: ImageKit - An Asynchronous Thumbs API