1. A method of determining based on image data of a shot image whether the shot image is a backlit portrait image, the method comprising: acquiring the image data; analyzing the acquired image data on a pixel-by-pixel basis to determine whether the shot image is a backlit portrait image; analyzing the acquired image data on a pixel-by-pixel basis to determine whether a ratio of number of dark outer area pixels having luminance not larger than reference luminance to total number of outer area pixels is not larger than a first predetermined value, the outer area pixels being other than inner area pixels among all pixels constituting the image data, the inner area pixels constituting image data corresponding to a central portion of the shot image; analyzing the acquired image data on a pixel-by-pixel to determine whether a ratio of number of bright outer area pixels having luminance not smaller than the reference luminance to the total number of outer area pixels is not smaller than a second predetermined value; and concluding that the shot image is a backlit portrait image if the shot image is determined to be a backlit portrait image, the ratio of number of dark outer area pixels having luminance not larger than the reference luminance to the total number of outer area pixels is not larger than the first predetermined value, and the ratio of number of bright outer area pixels having luminance not smaller than the reference luminance to the total number of outer area pixels is not smaller than the second predetermined value.