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!

Slight tangent, but have you considered using these new generative models like Dall E 2[1]? So far my experience has been that they generate great oil paintings. Maybe there could also be an automated pipeline that generates oil painting covers using book titles or first sentences.

[1] https://openai.com/dall-e-2/

Leaving aside the aesthetic considerations, the images produced from OpenAI software aren’t public domain, and so we can’t use them. See https://openai.com/api/policies/sharing-publication/ for a list of restrictions OpenAI places on the use of their generated images.