An image forming system has a plurality of image forming devices connected via a network. The system includes an image processor which processes an input image signal to produce a processed image signal. Respective additional information is superimposed on the processed image signal in a form that is not conspicuous to the human eye in preparation for transfer of the processed image signal to each of the plurality of image forming devices, with the respective additional information being different for each of the plurality of image forming devices. The processed image signal with the respective additional information superimposed thereon is transferred via the network to the respective ones of the image forming devices via the network.