1. An image forming apparatus comprising: an image combination circuit that combines a form image with an input image to generate an output image, and a form image acceptance circuit that performs a process of accepting the form image used by the image combination circuit, wherein the image combination circuit is able to combine the form image with the input image by means of a multiple form successive combination method in which in a case where there are a plurality of the input images, when a plurality of the form images are accepted by the form image acceptance circuit, a respective one of the form images accepted by the form image acceptance circuit is successively combined with a respective one of the input images, and the multiple form successive combination method is a method in which in a case where the input images to a number of three or more are used, when the form images to a number of three or more are accepted by the form image acceptance circuit, a respective one of the form images accepted by the form image acceptance circuit is successively combined with a respective one of the input images.