simple concept, but messy
When you dump a box of puzzle pieces out on the table, they're in a big pile you'll need to sort through. It's not so bad because you have a blank surface (like the table) behind it to do this quickly. In your game however, there's no "white space" for the eye to do the sorting quickly. In the easy version, if you know where a piece goes, you might not be able to place it because other pieces are in the way. In the hard version, it's just difficult to see the little pieces, and there's almost no room but the puzzle area itself to move things around.
I'd strongly suggest adding a "working area" to your game. Have like half the space for the puzzle, and half be empty area. It would be really neat if in the working area, you could put together puzzle chunks that could later be moved into the puzzle.