I just add Additional pages with the art.
There can only be one True (marked as) Cover, the rest are just (pages)
DO NOT add the images to the file flagged as Cover, it can (and probably will) get updated, orphaning the images (no use, subject to cleanup tools)
I simply name my files alt_cvr(# if more than 1), back_cvr (this is obviously a artifact from my paper copy. e-books have no backs

)