They need an oil painting that is submitted as a “link to a page scan of a book published before January 1, 1929 that reproduces the painting you selected”.
And yes, this is for US-Public Domain clearance. The cover art for an existing copy of the book has a separate copyright from the contents - creator lifetimes and what not, so they seem to be playing it safe.
And sticking to oil paintings give it a consistent aesthetic which I like.