For any WordPress users who might see this, you can define a responsive retina enabled featured image for your page in the header of the .md file like this:
title: 'My Grav Page'
Make an images directory at /user/pages/images, and then upload the image file to the directory. Make sure it’s an oversized image for retina displays, and give it the filename ‘email@example.com’
In your twig template, add the following to show the image:
Grav will now create and output the correct size image automatically. Once generated, the new images will be stored in the image cache. Neat.
You can add whatever custom variables you need, so the possibilities are endless, and very simple to manage.