1. An image processing apparatus, comprising: an image acquiring section implemented by a programmed computer device and configured to acquire one or more images; an image analysis information acquiring section, implemented by the programmed computer device and configured to acquire image analysis information on each of the one or more images; a theme information storage section implemented by the programmed computer device and configured to store information on photography tendencies of images in association with each of one or more themes; a theme determining section implemented by the programmed computer device and configured to determine a main theme representing a theme of each group of images related to each other among the one or more images and a subtheme representing a theme of each of the one or more images based on the information on photography tendencies of images associated with each of the one or more themes and the image analysis information on each of the one or more images; and a theme information output section implemented by the programmed computer device and configured to output information on the main theme and information on the subtheme, wherein the theme determining section comprises: a subtheme determining section configured to determine the subtheme based on the information on photography tendencies of images associated with each of the one or more themes and the image analysis information on each image of the one or more images; and a main theme determining section configured to determine as to which main theme a subtheme of each image of the group of images belongs to, based on the information on photography tendencies of images associated with each of the one or more themes and the subtheme of each image of the group of images, and determine a main theme to which a largest number of subthemes belong as the main theme of the group of images.