A method for processing image data comprises generating face data representing a set of detected faces from image data representing a set of images forming a library of images, using the face data to determine a sub-set of the images from the library in which at least the same person appears, and generating at least one clothing signature for the person representing clothes worn by the person in at least one of the images in the sub-set, and using the or each clothing signature, identifying further images of the person in the library.