1. A non-transitory computer-readable storage medium having tangibly recorded therein an image processing program for texture-mapping a three-dimensional polygonal model; the image processing program causing a computer to execute: reading, from storage locations, multilayer polygon data for constructing multilayer shell polygons including multiple layers of polygonal models; reading, from the storage locations, a shell texture image which is shared for use among the multilayer shell polygons and includes a plurality of areas having transparency different from one another; and texture-mapping each layer of the shell polygons with the shell texture image distorted to varying degrees depending on a position of each layer of the shell polygons, so as to position each layer, wherein each layer of the shell polygons is texture-mapped with the shell texture image distorted by referring to a value of a point shifted from a point to be originally referred to in the shell texture image, wherein the image processing program further causes the computer to execute reading a distortion map indicating a distortion amount of each point of the shell texture image, a distortion amount of each layer of the shell polygons is calculated from the distortion amount indicated by the distortion map, so as to determine, based on the calculation result, the point to be referred to in the shell texture image, and the shell texture image is distorted on a point-by-point basis within each shell polygon according to the distortion map, such that the amount of distortion depends on the particular point.