1. An image processing method for creating a layout image by arranging a plurality of images in a layout area where images can be arranged, the image processing method comprising: detecting a face area or face areas in each of the plurality of images; setting an all-face-inclusion area that includes all of the face areas detected in the face area detection operation for each of the plurality of images; setting a plurality of unit areas where the plurality of images are to be arranged by dividing the layout area on the basis of the number of the plurality of images and the layout area itself; selecting a maximum-face-number image, which is an image that includes the maximum number of the face areas detected in the face area detection operation, as a reference image among the plurality of images, adjusting the size of the reference image while keeping the horizontal-to-vertical ratio of the reference image within a range that ensures that the size of the reference image after the adjustment is not smaller than the size of a unit area set in the unit area setting operation and that the all-face-inclusion area set in the all-face-inclusion area setting operation is included in the unit area set in the unit area setting operation, and setting the size of one face area that is the largest among the face areas detected in the face area detection operation after the size adjustment as a reference face size; adjusting the size of each of the remaining images, which excludes the reference image, while keeping the horizontal-to-vertical ratio of the image within a range that ensures that the size of the image after the adjustment is not smaller than the size of a unit area set in the unit area setting operation and that the all-face-inclusion area set in the all-face-inclusion area setting operation is included in the unit area set in the unit area setting operation in such a way as to approximate the size of the largest face area included in the image to the reference face size; and creating a layout image by sequentially arranging the plurality of images at the unit areas set in the unit area setting operation with trimming at a border between the unit area where the image is currently arranged and an adjacent unit area where another image has already been arranged in preceding or previous arrangement processing, if any, besides trimming of any part of the image outside the layout area.