1. An image output apparatus, comprising: a first synthesis processing unit configured to read a plurality of designated input images from an external storage area storing the plurality of input images, generate a first synthetic image for display through a synthesis process of causing one of the plurality of read input images to be superimposed on any other one of the plurality of read input images, and output the first synthetic image; a second synthesis processing unit configured to generate a second synthetic image for synthesis through a synthesis process of causing the plurality of designated input images to be superimposed on any other one of the plurality of input images included in the first synthetic image and cause the second synthetic image to be stored in the external storage area; and an image selection control unit configured to instruct the second synthesis processing unit to generate the second synthetic image including the plurality of input images selected based on input image information that is input, select whether or not the second synthetic image is used as one of the input images used to generate the first synthetic image based on the input image information, and instruct the first synthesis processing unit to generate the first synthetic image including the plurality of selected input images.