First off the game looks really nice
How about showing the image whole and without any lines or other game UI when it's completed. Like a small reward?
Where are the images from? Do you have the right to use them?
Should you really have them unlock one at a time? It's not like the puzzles get any harder. It's a good thing to have a sense of progression in a game. But I wonder if it's not better to allow the player to choose between more puzzles. For example unlocking the top row to start and then unlocking puzzles column by column. So by completing a puzzle you unlock the one below? Allowing the player to choose between 3 puzzles in each category.