Missing a few QoL features (like undo and having the numbers in the hints grey out individually when the corresponding squares are placed down), but otherwise great game, easily up there with Pictopix and Picross Touch over on Steam.
I did the secret puzzles + collage in one sitting, most of the collage puzzles were super easy but I'm not complaining.
I appreciate that the puzzles never went larger than 20x20. 25x25 and 30x30 puzzles aren't hard per se but they quickly get obnoxious. I also appreciate that you did not fall into the trap of penalizing the player for making mistakes, as that inadvertently gives the player information they can use to reset and cheese the puzzle while also adding nothing positive to the experience.
Some traps you did fall into are symmetrical puzzles and the aforementioned collage. Whenever the hints along one axis are symmetrical, you can actually simplify solving the puzzle. For example, if the top hints are all symmetrical and you see, say, [1 5 1] in the row hints, you can just fill in the middle 5 squares and be guaranteed that it's correct. This heuristic has never failed me. The collage is a trap because being part of a larger image means that most of the puzzles will be caught in a round shape or flat line, resulting in several full/empty rows and columns, or in one puzzle, it was all 10s so I didn't even have to solve it because there was nothing to solve. Again, not complaining. The super easy puzzles feel like a little reward for all the hard work so burning through them rapidly is cathartic.
Another trap to watch out for is that some nonogram implementations like to check your solution against the map the puzzle was generated from instead of against the hints, resulting in valid solutions being rejected because the puzzle has multiple solutions but the game only accepts one. I don't know which method yours uses, as you managed to avoid such situations, so great job on that. Puzzles with multiple solutions typically happen when the source pixel art has dithering.
Overall, very polished, I had a blast. My overall time was 25:29:06, though itch will not let me paste the image, apparently.
Edit: Maybe if I try uploading...

So uploading works but not pasting.



