Github unfortunately refuses to fix that issue but doesn't inform me that all images + concepts i post in Github Issues (private repository) are public for everybody (i know the urls are cryptic, but whoever catchs them can visit them everytime without my password)
Reply by Github:
We may change this functionality in the future, but do not have anything to announce now.<p>Helllo? - even an old vBulletin Forum sends user's content through a permission proxy!