1. An image-processing device comprising a processor, the processor being configured to: acquire a radiation image of an imaging subject captured by a radiation image capture device for radiographic imaging; acquire body thickness information indicating a body thickness of the imaging subject in a direction in which radiation passes through; and add to the radiation image an assist line image that would be expected to be obtained if a predetermined member had been disposed and captured at a position inside the imaging subject determined based on the body thickness information, or at a position between the imaging subject and the radiation image capture device.