I enjoyed all the ones after the "you win" one. I found them challenging in a fun way. I think the hardest one was the cat. I went into it thinking it would be an easier one and was delightfully surprised at the challenge. Unlocking new, harder levels after completing the base game makes sense, though I wonder if you want to have the 'you did it' there include a comment about beating the base game and unlocking bonus challenge levels.
Puzzle games are some of my favorite types of games, and I like these type of pictures, so even the harder puzzles were a lot of fun for me.