I've thought about this some more after reading Noah's posts (and replies) on this subject. "Fair use" is a subjective concept, usually applied to academic research, e.g. quoting a passage from a copyrighted text for scholarly purposes. A teacher cannot take a copyrighted poem, for example, and make 30 copies then pass them out to a class.

In the case of an album cover, anyone posting a photo is essentially reproducing the entire work.

My final take:

WOULD the copyright owners of the Dark Side of the Moon cover sue me if I posted my photo of my album on Medium? Probably not.

COULD the copyright owners sue me? It's certainly possible. Remember that Disney sued an ordinary citizen for making a videotape copy of one of their films from a broadcast.

WOULD such a lawsuit result in charges against the Medium author? Who knows? There's going to be a major case in the Supreme Court over an Andy Warhol artwork. No telling how that will turn out. But unless the Medium author has legal insurance, the suit would be prohibitively expensive.

So I'm deciding not to use any more photos of album covers.