The images aren't the same size. A big spider blocks at least one of the differences. These are fundamentally huge problems that you've managed to put into a game in which the only form of player interaction is to click on parts of a pair of images.
As for level design, in every "difference" something's simply missing from one image or the other. There's never a case of something existing in both images but just being, ya know, DIFFERENT. So all the levels feel incredibly same-y and very easy. Rather than a missing eye in a carved pumpkin, have one eye be a triangle and the other a circle. Instead of yet another missing bat, have the bat fly upside-down. Have a window change color between images instead of just not being there in one. etc