The Hidden photos are pictures of Om Nom scattered throughout Cut the Rope: Experiments. Player sees them as sheets of paper stuck to the background of the levels. Tapping them reveals a picture of Om Nom, usually connected with the theme of the level, such as meeting a robot or having a bath.
Below is a list of photos found in Cut the Rope: Experiments.
Note: finding photos requires a lot of perseverance, as they are mainly located at the edges of the screen and are hard to spot.