An electrical-device-implemented image coding method includes: a raw block is obtained from an image. There are a max pixel value and a min pixel value among the raw pixel values of the image. A first difference sum, which is sum of differences between the raw pixel values and the max pixel value respectively, are calculated. A second difference sum, which is sum of differences between the raw pixel values and the min pixel value respectively, are calculated. If the first difference sum is not smaller than the second difference sum, each of the residue values is calculated by subtracting the min pixel value from the corresponding raw pixel value. Otherwise, each of the residue values is calculated by subtracting the corresponding raw pixel value from the max pixel value. Each of the residue values is encoded into a compressed codeword. A bit stream is generated according to the compressed codewords.