1. An image generating device comprising: a human detector implemented by a processor that detects a human from a captured image; a motion detector implemented by a 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 a processor that temporarily 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 a camerawork between the frames; a cut-out frame corrector implemented by a processor that interpolates the cut-out frame to reproduce the new image smoothly; and a composition information shaper implemented by a processor that cuts out the new image from the captured image by using the cut-out frame interpolated by the cut-out frame corrector, and converts a size of the new image to a predetermined size.