A tile image including a predetermined number of pixels (e.g., 32×32 pixels) is divided into blocks (pieces) of a size of 2×2 pixels each to compare color data of each pixel in each piece. A pattern flag indicating an arrangement pattern of the color data contained in each piece is identified, and first color data and color data other than the first color data in each piece are identified, where the first color data is color data corresponding to a pixel at a predetermined position in each piece. If it is determined that all the pieces, except for a piece consisting of the same color, included in the tile image consist of the same color, the two colors are set to representative colors for reference in the tile image.