Is there some reason against using the original cover art if they're preserved somewhere? Surely the cover art doesn't have a longer copyright?

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”.

Section 10.3.3.7 of their Manual: https://standardebooks.org/manual/1.8.0/10-art-and-images

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.