The historical criteria is fine-art style oil painting. These days we’re starting to use first-edition cover art a bunch for more modern productions if it’s good quality. We also tend to use abstract oil paintings for sci-fi.[1] Obviously, all art is sourced from the public domain too. We’ve also started a database of confirmed-US-PD artwork that we can use for future productions.[2]

[1] https://standardebooks.org/subjects/science-fiction

[2] https://standardebooks.org/artworks