I love the photos and the creativity that went into the game.
I think the most rewarding change for the player would be to reveal the differences that they missed at the end of the level, so that they could grow as a player and know what kinds of things to look for in future levels. Otherwise the game becomes mentally tedious. Intermittent reward is key to addictive games, and hiding the solution takes away that reward - just by ending the level, giving the player the satisfaction of seeing the other differences provides mental closure.
The subtlety of difference made the game too difficult. I was tempted to take screen captures into photoshop and overlay them, but that cheat was too tedious. At one point, I thought I'd spotted an extra hot air baloon, but it was only a speck of dust on my screen.
Also, the fonts are painful. Stick with something sans, classic and limited in color. You want people reading the text without mental engagement, not having to decipher it. Technology's gift of gradient type with a black outline is a gift, not be be abused.
Great work, but the nuance is just too much.