For everything except photos, I have started using .svgs instead of .pngs. They are resolution-independent, you can edit colors and strokes via CSS, and even include animations (check out the stars in the background of http://catnap.co). You can compress them so the file size is often smaller as well.
For retina.js I believe it should swap out images automatically as long as you link to the un-prefixed image. (E.g. display image.png but include image@2x.png in the directory and retina.js will take care of the rest.) I may be wrong but I don’t remember needing to work any Kirby magic to display retina assets.