Is it currently possible to specify a Retina image with the current GitHub-flavoured Markdown? Or an image size, which would let standard resolution devices scale the resource?
It's not a big deal, but it would add a nice touch.
Markdown is a strict superset of HTML, so you should be able to just make the HTML tag explicitly and set the size how you want. See http://www.mediacollege.com/internet/html/image-tag.html.