1. A method for operating an image-processing device, in which an image signal is digitally transmitted between a transmitter and at least one receiver, and the image signal comprises a timed sequence of images, wherein an image buffer is assigned to the receiver, into which successfully transmitted images are written, and the image most recently written to the buffer is repeated if a defect in the image transmission process occurs, wherein an alternating signal which alternates at a signal alternation frequency is superimposed on the image content of the images before they are transmitted and before the image buffer, and the signal alternation frequency is greater than or equal to the perception frequency resolvable by the human eye.