1. An image generating device comprising: a human detector implemented by a processor that detects a human from a captured image captured by an imaging system; a motion detector implemented by the processor that identifies a motion between frames from the captured image, each of the frames being a unit of an image; a cut-out frame decider implemented by the processor that decides a cut-out frame for cutting out a new image from the detected human and the identified motion based on a constraint condition regarding camerawork between the frames of the imaging system during capturing of the image; and a composition information shaper that cuts out the new image from the captured image by using the cut-out frame, and converts a size of the new image to a predetermined size.