This is pretty good

It takes me back to the good old days when I would get these puzzles in elementary school and would get extra credit if I found all of the differences :)

I just have one suggestion. Maybe you can add in a "hints" section where the person gets two or three hints throughout the game to keep it moving along.

I can't find 5

I can't actually find 5 differences....

I did notice that they change, so this idea has potential. I'll also say maybe you should give the option of a changing or pickable soundtrack, cause that one gets annoying fast. I would suggest also adding more levels, maybe making them harder? Start with bigger details, and progress to smaller ones?