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...
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?