1. An image synthesis apparatus, comprising: image selection means for selecting two or more three-dimensional images to be synthesized from among a plurality of three-dimensional images; order determination means for determining, based on parallax amounts of the selected three-dimensional images, a synthesis order representative of an order in which the selected three-dimensional images are to be synthesized; image synthesis means for synthesizing the selected three-dimensional images in accordance with the synthesis order; and control means for controlling said image selection means, said order determination means and said image synthesis means in response to an operation of a user.