Cover art must be in a fine art oil painting style, and it's up to each producer to find something thematically appropriate. Once they do we require proof of US public domain status. See https://standardebooks.org/manual/1.6.4/10-art-and-images#10...

Wikiart and Artvee are good places to start cover art research. Many museums now have explicit CC0 collections, too.

Finding good public domain cover art can be extremely time consuming - it's the part of the process most likely to make a new contributor give up. But when you find a great cover, it feels great!