1. An image processing device, comprising: an image inputting part taking in at least one of a plurality of low-resolution images and a high-resolution image obtained by shooting the same subject while changing exposure condition; a shift detecting part detecting a positional shift of pictorial pattern between each of the plurality of low-resolution images and the high-resolution image; and a gradation expanding part generating a synthesized image in which a range of reproduced gradation is expanded by performing position alignment between each of the plurality of low-resolution images and the high-resolution image based on the positional shift, extracting gradation information of the plurality of low-resolution images, and synthesizing the plurality of low-resolution images with the high-resolution image.