GitHub itself has a “secret” feature to upload images.
I read about this in a comment by GitHub’s own Phil Haack:
I edit (or create) an issue and drag it into there and copy the resulting markdown into my post. It’s probably an abuse of GitHub issues.
If you do it like this, the image will be stored on some GitHub server, and it will have a URL like this one:
https://f.cloud.github.com/assets/19977/1656110/a3f8b280-5b6d-11e3-818d-c06ab05bd613.jpg