1. An image generating apparatus, comprising: a diagnosis image generating section that generates, as a diagnosis image for every first time interval, a moving image in which a predetermined part of a human body or an animal is photographed, wherein said diagnosis image generating section comprises: a moving image acquiring portion that acquires said moving image; and a subtraction image generating portion that generates a subtraction image between first frame images and second frame images in said moving image acquired in said moving image acquiring portion, wherein said subtraction image is generated by having said subtraction image generating portion subtract image data of said first frame images from image data of said second frame images, and wherein said diagnosis image is said subtraction image; an image target setting section that consecutively acquires a first image of said diagnosis image at a predetermined time and a second image of said diagnosis image after a second time interval from said predetermined time, wherein said second time interval is determined from said moving image and said second time interval is longer than said first time interval; a pixel color converting portion that converts, of pixels of said first image of said diagnosis image and said second image of said diagnosis image, colors of pixels satisfying a predetermined condition to be distinguishable; and a display image generating section that generates a third image for display using said first image of said diagnosis image and said second image of said diagnosis image whose colors of the pixels have been converted by said pixel color converting portion.