Here are some examples that I think really encapsulates this:

Caravan - In The Land of Grey and Pink

Whimsical, fantasy-like, psychedelic, a paradoxical mélange of upbeat and melancholy.

Abstract, surrealist, futuristic-yet-earthy, all tied together with an African flare. 

Pastoral, dreamy, idealistic, nostalgic.

Intricate, pestilent, cold, technical, morbidly fascinating.

Those are the first few prog examples that leap to mind. There are also several non-prog covers that I immediately thought of that I could also share.

Relayer

Tales from topographic oceans

A trick of the tail

Selling England by the Pound (the lawnmower on the cover was painted in deliberately because of the song "I know what I like.")

Atom Heart Mother (I know this is an odd one but those cows on the back cover are kind of spooky like some of the music and the front cover depicts the rural and pastoral nature of some of it too).

That's all I can think of actually.

The Yes and Genesis ones are great examples, too. Actually, I think that any of the Genesis covers from Trespass to Wind & Wuthering are actually fairly good representations of the music, though the fact that Paul Whitehead often incorporates lyrical elements into his paintings probably helps. 

great imagery, from the keyboardist, I believe.

I agree with the other Genesis albums you mentioned too. I would add And then there were three but I really am not sure what the heck that is suppose to be. Wind and Wuthering is a good example and so is Trespass. The others less so but even those do fit to some degree.

Also, I think Jethro Tull's songs from the wood is a good one for this too. The carved out record player on the back could literally be a song from the wood if it could play.  The first King Crimson album cover(the red face guy) fits the music in my opinion. The other ones not so much. Apparently there is more than one cover for Islands but the one I am familiar with which is some kind of astronomy picture sort of fits.

When I close my eyes listening to the titletrack the whole battle that's displayed on the albumsleeve, comes to life in my head.

Debut album by German band Dunst. The music fits the cover nicely.

same goes for the complete wrap around morbidity of COMUS' first utterance. The twisted black and white figure on the cover depicts the dark and macabre subject matter with a twist of suffering intermingled in every nook and cranny

Funny that you mention that last point, because I think In The Court of The Crimson King is actually the only early King Crimson cover that doesn't fit the whole album, actually. It definitely matches the opening track (as it should, because the screaming man on the cover is the 21st Century Schizoid Man), but I feel like the rest of the album demands something a bit darker, colder, and more passive as far as cover art. A woods in the nighttime kind of thing, perhaps.

As for In The Wake of Poseidon, I feel like the blend of colours mixing from one to another do a very good job of representing the way that the album flows between all its extremes. The deep red hues of Pictures of A City weaving into the light, tranquil blue of Cadence and Cascade, etc. 

And Lizard is probably the best matched to the album's contents, since the scenes are pulled directly from the lyrics. More generally, though, the cover art feels almost like a tapestry, which captures the album's complexity and intricacies, while also capturing some of the medieval, fantasy allure. 

Islands is a weird one because the Trifid Nebula painting really shouldn't make me think of the music at all, but it all makes sense in my head. 

Larks' Tongues In Aspic is another great one as far as I'm concerned. The ambiguous nature of it, the hippie-esque imagery, the coy expression of their faces; it feels like a gateway into the occult. Which, by the time Cross comes in with his violin ostinato in the opening track, you darn well know it is!

Love that one! I also find Eat A Peach to be a perfect cover for the music. Apparently it was designed with "and early-morning-sky feel" in mind, which I think they captured flawlessly, as did the band with tracks like Mountain Jam and Blue Sky.