A system and method for determining if a computer user is a human or a computer program. A computer user is presented with a series of images, the series of images including: at least two related images of an object, at least one of the at least two related images independently processed using an image modifying algorithm; and at least one unrelated image. The computer user is then requested to identify the related images from the series of images. These image identifications are received from the computer user. A determination is made that the computer user is: a human if the image identifications only identify the related images from the series of images; and a non-human if the image identifications do not only identify the related images from the series of images.