1. A computer implemented method of generating a game comprising: receiving a user-selected original image; storing the image in memory; from the original image, automatically generating a pair of images which are visually identical except for a predetermined number of random localized modifications, the game being played by spotting differences between the pair of images as a result of the modifications; and providing the pair of images to the user as a game; wherein at least one of the localized modifications includes insertion of an object and the method comprises: automatically classifying each of a plurality of regions of the image into a respective class for the region; and accessing a library of objects which are each indexed according to the region classes with which the object is compatible and, for at least one of the classified regions, randomly selecting an object that is compatible with the region and applying the object to the region.