In a monitoring method and image processing apparatus for monitoring electronic transmission of an image, an image detector supplies an input image to a processing circuit that caches it in an image storage. The cached image, or an image derived therefrom, is forwarded as an output image to a viewing device, which outputs the image to a viewer. To monitor the image transfer, an insertion circuit upstream from the processing circuit inserts a dynamic input test signal into the input image. A test circuit downstream from the processing circuit tests whether the output image contains a dynamic output test signal corresponding with the input test signal. Given the absence of the dynamic output test signal, alarm notification is provided to the viewer.