It became predictable
After the first three challenges, I noticed that the game became considerably easier where I was finding all the differences in less than a minute. Highest bonus time was +99. That's because the 'differences' were nearly the same each time. Spotted/Striped sleeves, Tattoos on the Elf, missing foreground object, missing background object, ooh a missing tree limb, and done!
5 of the differences were almost always something to do with patterns on the clothing or skin that were not meant to be there. Just too easy (and I played on normal) I would like to see more creative differences in the photos. Something like a rearranged shadow, or a missing chair leg that is just obscure enough to go overlooked.
The art was nice and you had good music. But because the game was suspiciously too easy, I began to wonder if you simply used the game as an excuse to showcase your art.