Am I standing in front of the camera or not? The face illusion is induced by cutting out a frontal photo along the lines of a profile picture and revealing some persuasive side-elements from the second picture. The image below shows which photos were used for the trick. The hardest parts are in my opinion the eyes and the teeth. So if you want to do it yourself, keep your mouth closed and throw away any glasses.
It is inspired on some other profile-illusion pictures where different techniques were used. The first one is made by Jesús González Rodríguez and also uses two pictures to create the illusion. The second one (by Flickr artist Kennysarmy) uses only one photo. The effect is achieved by adding and removing shadow at certain spots.