I found this answer where some of the
i.stack.imgur images were "no longer available", so I converted them to
i.imgur. Discussion on imgur.com is deleting images makes it sound like this shouldn't happen any more and that it's better to keep i.stack.imgur.
It's bad to have answers with invalid pictures, but it sounds like we should always use i.stack.imgur.
What should I do? Is there a place to report broken images?