1. An image processing apparatus comprising: subject region detection means for detecting regions of subjects included in an input image; cropped region setting means for setting a region including all of the regions of the subjects detected by the subject region detection means as a cropped region; expansion region addition means for adding an expansion region to expand an end of the cropped region only by a predetermined width; cutting means for cutting a cut image formed from a cut region including the cropped region and the expansion region from the input image; subject adaptation size changing means for changing a size of an image formed from the cut region by deleting a region where an influence on the subjects is small in the cut image; and an entire image size changing means for changing the size of the entire image changed in size by the subject adaptation size changing means at a predetermined ratio, wherein the subject adaptation size changing means includes input image energy map generation means for generating an input image energy map from the cut image based on energies of adjacent pixels of the cut image, reduced energy map generation means for generating a reduced energy map corresponding to a reduced image, which is formed by setting one block formed by a plurality of adjacent pixels of the cut image as one pixel, by reducing the input image energy map, reduction seam search means for searching a reduction seam formed by binding pixels of a path along which a cumulative energy value is minimum among a plurality of paths formed from a pixel located at one end of the reduced image to pixels at the other end of the reduced image sequentially via the pixels adjacent in a direction of the other end in the reduced energy map, partial seam search means for searching a partial seam formed by binding pixels with a minimum cumulative value of the energies of the pixels among a plurality of paths formed from a pixel located at one end, which includes an end of the reduction seam searched by the reduction seam search means, of the cut image in the input image energy map to pixels located at the other end of the cut image sequentially via the pixels adjacent in a direction of the other end, and reduction means for reducing the input image by deleting the pixels forming the partial seam searched by the partial seam search means from the input image.