An image processing device (1, 2) comprising: a thumbnail selecting section configured to select, for displaying a thumbnail, any of first thumbnail image data generated by applying resolution conversion to panoramic image data generated by combining a plurality of captured images, and second thumbnail image data generated by performing aspect ratio adjustment processing and resolution conversion on the panoramic image data, the first thumbnail image data and the second thumbnail image data being for displaying a panoramic image at a reduced size; and a display data generating section configured to generate display data for displaying a thumbnail with any of the first thumbnail image data and the second thumbnail image data in accordance with selection of the thumbnail selecting section, wherein the second thumbnail image data is generated by performing the aspect ratio adjustment processing on the entirety of the panoramic image data in a manner that retains information from the entire panoramic image by performing one selected from the list consisting of: a) texture-mapping the panoramic image data to a three-dimensional model of a re-projection surface and performing rendering for projection from a viewpoint position overlooking the model from the outside; b) applying polar coordinate transformation on the panoramic image data; and c) determining the importance of pixels in the panoramic image data using an energy function and preferentially removing pixels in a manner that minimizes energy loss and as many as possible pixels of high importance are retained; wherein the thumbnail selecting section selects any of the first thumbnail image data and the second thumbnail image data in accordance with one or more of an aspect ratio of a display area of a display device that supplies the display data, a size of a display area of a display device that supplies the display data, a number of pieces of thumbnail image data included in the display data, and a posture of the display device that supplies the display data. a thumbnail selecting section configured to select, for displaying a thumbnail, any of first thumbnail image data generated by applying resolution conversion to panoramic image data generated by combining a plurality of captured images, and second thumbnail image data generated by performing aspect ratio adjustment processing and resolution conversion on the panoramic image data, the first thumbnail image data and the second thumbnail image data being for displaying a panoramic image at a reduced size; and a display data generating section configured to generate display data for displaying a thumbnail with any of the first thumbnail image data and the second thumbnail image data in accordance with selection of the thumbnail selecting section, wherein the second thumbnail image data is generated by performing the aspect ratio adjustment processing on the entirety of the panoramic image data in a manner that retains information from the entire panoramic image by performing one selected from the list consisting of: a) texture-mapping the panoramic image data to a three-dimensional model of a re-projection surface and performing rendering for projection from a viewpoint position overlooking the model from the outside; b) applying polar coordinate transformation on the panoramic image data; and c) determining the importance of pixels in the panoramic image data using an energy function and preferentially removing pixels in a manner that minimizes energy loss and as many as possible pixels of high importance are retained; a) texture-mapping the panoramic image data to a three-dimensional model of a re-projection surface and performing rendering for projection from a viewpoint position overlooking the model from the outside; b) applying polar coordinate transformation on the panoramic image data; and c) determining the importance of pixels in the panoramic image data using an energy function and preferentially removing pixels in a manner that minimizes energy loss and as many as possible pixels of high importance are retained; wherein the thumbnail selecting section selects any of the first thumbnail image data and the second thumbnail image data in accordance with one or more of an aspect ratio of a display area of a display device that supplies the display data, a size of a display area of a display device that supplies the display data, a number of pieces of thumbnail image data included in the display data, and a posture of the display device that supplies the display data.