Does anyone know how easily you can do "copyright clearance" for these supposedly public domain images?
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?
Just contact the original author. You may need to pay extra for the Medium.
Search for public records about the image. If you can confirm the author is dead for at least 70 years (in case the author originally held the copyright) and the work is at least 120 years old or was published more than 95 years ago (in case this was work for hire) you are good
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