These folks seem to be more up-front about the issue than many sites I’ve seen:
https://pdimagearchive.org/reusing-images/
> On each image page we communicate to the best of our knowledge the rights status of both the underlying work and the digital copy of this work. We provide this information based on a basic knowledge of copyright law and what is communicated by the source institution — it is strictly meant as a guideline and it should not be taken as legal advice. We admit no responsibility for any untoward consequences that may arise through reuse of material featured on our site. If you are requiring certainty as to usage allowed for an image, then you are encouraged to check with the source institution and make your own investigations.
https://standardebooks.org/artworks/arthur-i-keller/calvin-c...
On the other hand they do allow search by century and very little from the 19th (and none earlier) century is still in copyright anywhere.
There is a possible problem with countries where a photographer can have copyright over their photograph of an earlier work. Again, there is not global standard.
To be specific, Germany is one of the few countries where this applies https://en.wikipedia.org/wiki/Threshold_of_originality
It's an absolutely stupid idea IMO.
Adding this to the list for one of my side projects[0].
Edit: I saw the small link to examples :)
For example, the page for the first image I clicked on said:
Date
1833
Underlying Rights
Public Domain Worldwide
Digital Rights
No Additional Rights
* Source states “no known restrictions”
* We offer this info as guidance only
If, for example, you design the cover of a self-published book around such an image, is Amazon KDP going to reject it, because they don't accept that screenshot as sufficient proof of rights?
For younger works other conditions might make a work public domain, like the work being created by a US federal government employee as part of their duties
For example, I saw a writer-artist complain that they couldn't show the big ebook store proof that they'd licensed the art for their cover, since they painted it themself, and there was no documentation licensing it to them.
So one could use practical advice from someone who's figured out the rules, and has the battle scars, like: "With public domain art, just make sure you do X and avoid Y; otherwise, there's a 50% chance that a random copyright check the first week will result in your book temporarily being shadowbanned, and you won't get the crucial early sales while the algorithm is trialing exposure of your book in searches and categories, and then the algorithm will pretty much never show your book to anyone ever again."
https://commons.wikimedia.org/wiki/Help:Public_domain
https://commons.wikimedia.org/wiki/Commons:Freedom_of_panora...
https://commons.wikimedia.org/wiki/Commons:Reuse_of_PD-Art_p...
https://commons.wikimedia.org/wiki/Commons:De_minimis
Of course sometimes its just a risk trade off, and depends more on how the publisher feels then what the actual rules are.
You don't want a book's carefully choreographed launch sabotaged by various companies' Kafkaesque bureaucratic processes, while you're trying to finesse the algorithm and the publicity you've scheduled.
(I should've said upfront that I know a bit about public domain and copyright law as a layperson. And I also know that I can't necessarily point a CSR to law surveys if something gets flagged. I need to know the "do X and avoid Y" that is proven to actually work smoothly with all these companies' ideas of "copyright clearance", so that I hopefully never have to talk to a CSR.)
The entire US economy wouldn't be banking on AI if they were just going to let some patreon furry pornographer win a landmark case against the top 5 biggest capitalized company don't you think?
I'll let you keep dreaming if it makes you happy.