[Formendacil]

Even if oil painting on canvas would have been a bit anachronistic in a pre-Renaissance setting, that doesn't mean that visual flat images would have been out of place, as has been noted. Tolkien's comparison of Gondor to the Byzantine Empire makes me think immediately of mosaics. For that matter, Tolkien's comparison of the Númenor to ancient Egypt also makes me think of the rather famous Egyptian 2-dimensional art. Obviously, this doesn't imply an artistic connection, but it certainly removes any charges of anachronism.