It's been proven that dogs recognize their owners, but exactly how they do that is still unknown. Is it visual? Smell? Posture? Routine? To narrow it down, a team at the University of Helsinki created a study to find out if dogs can recognize their owners or other canine members of the family just from a photograph. Most animals can't do this; they don't recognize the connection between a photo and a real-life object or person.
So far the specialized skill for recognizing facial features holistically has been assumed to be a quality that only humans and possibly primates possess. Although it's well known, that faces and eye contact play an important role in the communication between dogs and humans, this was the first study, where facial recognition of dogs was investigated with eye movement tracking.
The experiment measured the amount of time the dog looked at each picture. It found that the dogs looked longer at pictures of dogs than at pictures of people, but that they also looked longer at pictures of people they knew than people they did not.