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
with a link to: https://pdimagearchive.org/reusing-images/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?