1. A computer-readable storage medium having stored therein an image processing program for texture-mapping a three-dimensional polygonal model; the image processing program causing a computer to execute: a shell polygon reading step of reading, from storage means, multilayer polygon data for constructing multilayer shell polygons including multiple layers of polygonal models; a texture reading step of reading, from the storage means, 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 a texture mapping step of 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.