This is true, though there is a workaround by using cdn.saw fit.com domain. I use the trick to display SVG figures in these white paper notes[1], and I found out about it in this issue thread[2]

Example:

<img src="http://cdn.rawgit.com/<username>/<path>/<to>/<image.svg>">

[1]: https://github.com/samjabrahams/tensorflow-white-paper-notes... [2]: https://github.com/isaacs/github/issues/316#issuecomment-113...