We recently started using Pixelapse (we're a development team that just welcomed our first full-time graphic designer a few months ago). It's pretty nice, but there is one huge difference between design and development that makes the idea of "Github for design" seem like the wrong idea.
Developers really only need to share code with other developers. There's almost no such thing as a passive receiver of code. The ability to easily collaborate on code with other developers and within a team, via cloning, forking, pushing and pulling, makes Github (and Bitbucket and Gitlab, etc) immensely beneficial.
Design interactions, on the other hand, tend to be made up primarily of designers sharing designs with non-designers, whether it be the client for feedback or the technical team for implementation (or the printers, or the marketing department).
I think Pixelapse is more like the "Github for design" if you just think of Github as a 3rd-party repository for easy version tracking and uploading of code, and which makes it easy for others to comment and compare different versions of the code. In other words, minus the forking and cloning concept. At least, that's how we use it.