Thank you for all the feedback! It really does help.
Least-solved puzzles are:
#1 castle/zebra
#2 crying/beluga
#3 tie between instruments/logo, sun/zombies, and woman/chess
and from there it's a pretty big leap to #6
A lot of players have asked for a hint system. Wanting a way to get un-stuck makes a lot of sense. Locking in individual words might work, or might lead to people spamming words instead of thinking. It might dilute the satisfaction of getting the whole phrase at once. I'll have to try it and see how players feel.
I'm more inclined to give players a way to bypass puzzles they're stuck on. For example, solving extra paintings could give a key that could be used to open a lock later, then reclaimed if the player eventually solves another painting in that room.
The paintings with the castle/zebra have a few problems, including that the animals in front should more clearly be yaks and not some other kind of bovine (thereby showing that the zebras are both the last of their kind and the last in the alphabet. The appearance of the castle does not really look old enough, or foreboding enough, to suggest either of the words it's supposed to. And in general, I had hoped to each players that by this point in the game the room's theme (endings) could be useful clue in itself, but I failed to draw player's attention to the room or its theme on a consistent basis.