And you can’t delete uploaded images without contacting support.
- Go to a private GitHub repository.
- Create a new issue.
- In the issue description or a comment, insert an image.
- Save the description/comment.
- Now click the image – it will redirect to the raw image URL in your browser.
- Copy the URL and share it with your friends – it’s public.
- Now delete the issue or comment with the image.
- The URL is still public.
I reported this to GitHub support, and while they agree it is probably not optimal, they do not have a plan to change the behavior (as of 4/11/2017).