Under rating threshold (hideshow)
On the level where they enter the castle, and see the skulls and other treasures sitting there, one of the rows of gems around one of the other treasure is different, yet cannot be clicked on (counts as a mistake) and does not count for the level. Not good, need to make sure things like that don't slip through.
Under rating threshold (hideshow)
Some of your differences actually get covered up by elements of the UI. Example, on the running through the crags picture, the possible difference of the boy and dog's shadow is covered up on the bottom by the hint button overlapping the screen. I'd just make all the UI stuff normal size, and never allow it to intrude into the picture. Also had trouble on level one with the little red dot for changing sound/etc. covering up a star difference. So... yeah - you need to not have any part of your user interface whatsoever intruding onto the pictures in a spot the difference game. Anything else just WILL cause problems - as it has here.