There are lots of sites that provide images that somebody has claimed are public domain. But for significant use, you what you really need is provenance documentation.

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.

Standard Ebooks has a database of public domain oil paintings for use as book covers. SE is strict about copyright clearance and requires either scans of a copyright‐expired publication containing the painting, or an explicit statement from a reputable museum that the work is CC0. Here’s an entry I contributed:

https://standardebooks.org/artworks/arthur-i-keller/calvin-c...