1. A computer-implemented method comprising: obtaining training data for training a machine to output a threshold value for each of multiple facial features, the training data including (i) multiple facial images and, (ii) for each facial image, an indication of whether the facial image is of a picture of a person in front of a camera or of a person actually positioned in front of a camera; training the machine using the (i) multiple facial images and, (ii) for each facial image, an indication of whether the facial image is of a picture or of a person actually positioned in front of a camera; determining, using the trained machine, the threshold value for each of the multiple facial features; and using one or more of the threshold values for one or more of the facial features in determining whether one or more subsequently received facial images are of a picture of a person in front of a camera or of a person actually positioned in front of a camera.