Once you give someone the ability to see or hear something, there is no guaranteed way to take it back.
I say this because: "You revoke access to anyone in the future that even if they have the image, without the privilege, the file is useless"
In the same respect, you can't really stop people from taking screenshots or take a photo of... a photo. What we do is we put a watermark of the recipient in the image. This way, if that image get released anywhere, you knew who was the culprit and you can take further actions.
No box is ever impenetrable, then the next best thing is to know when, what, why, whom of the penetration. I hope that helps. It is a difficult problem to solve, however, we have to do something!
Thank you for your thoughts.
-V.