Some ideas for improvement
Its quite a good game, but not really an original idea.
For the moment, lets just blow unoriginality out of the matter and focus on the gameplay itself.
Very challenging, and I like that in puzzling games such as these, but the challenging levels plummet to zero once you realise you can click everywhere as fast as you want to find the differences with no setbacks.
Perhaps you should add a space beside the stars that fills with crosses everytime you click somewhere that doesn't have a difference. Perhaps, like the stars although ascending in number and not descending, it should get to 5 crosses and present a 'game over' screen, or just restart the game altogether.
A time-limit (or shorter if there is one, I never hit it) would be good too.
Good try though. Just fix the few minor errors and tweak the mistakes to make it that much better.