What is hidden in 3D image?

Little Johnny's kindergarten class was on a field trip to their local police station where they saw pictures tacked to a bulletin board of the 10 most wanted criminals. One of the youngsters pointed to a picture and asked if it really was the photo of a wanted person. 'Yes,' said the policeman. 'The detectives want very badly to capture him.
'Little Johnny asked, 'Why didn't you keep him when you took his picture?’