But you can't avoid the inevitable consequences of introducing a rule. If you don't intend to apply it retrospectively then posters of current material have a legitimate grievance. If you do intend to apply it retrospectively then much current material will have to be removed. Not applying a rule properly and comprehensively makes that rule an unfair and unjust nonsense.
And as I asked previously, what about information that is no longer available? You're not the only one adept at research, and my knowledge and memory of the entertainment and fashion industries is fairly extensive but even I struggle with and even fail to complete many identifications. So we would have to prohibit the posting of and the removal of material the information of which cannot be determined. That is the inevitable consequence of introducing a hard and fast rule.
Again, this is community forum. Not Getty. It is ar better to lead by example and demonstrate a better way and a more helpful habit. Not everyone will follow but more will than won't.
I note that you wrote "I can give instructions on how to easily determine even ancient pictures", and yet rather than do that straight away in the Discussion forum, explaining how much easier it would make finding material and determining duplication, you come to this section and propose a rule that everyone must obey.
How "respectful" and un-dictator-like.
BOC