1. An image processing device, comprising: a processor which is configured to: control an image sensor to successively capture a plurality of images; partially superimpose the plurality of captured images to generate a synthesis image such that an additional one of the plurality of images is added to the synthesis image generated at that point, each time the image sensor captures the additional one of the plurality of images; designate a composition which specifies a size of a composition region and a position of at least one predetermined feature region within the composition region, the composition region being an image region of the synthesis image to be generated, wherein the processor is configured to designate the composition before generation of a final synthesis image is completed; successively judge, after an additional one of the plurality of images is added to the synthesis image, whether the synthesis image generated at that point contains the composition region having the size specified by the designated composition and including therein the predetermined feature region at the position specified by the designated composition, wherein the composition region is larger than an image region which can be acquired by a single image capture by the image sensor, and the composition region includes image data of the synthesis image corresponding at least partially to portions of the plurality of captured images which are superimposed with each other and at least a part of a most recently added additional one of the plurality of images which is not superimposed; and control a predetermined processing to be executed when it is judged that the synthesis image contains the composition region having the size specified by the designated composition and including therein the predetermined feature region at the position specified by the designated composition, using the synthesis image generated at that point as the final synthesis image.