Provided are an image compression method and apparatus for differentially encoding digital image signals based on a Joint Photographic Experts Group (JPEG) image compression standard. RGB image signals are converted to luminance and chrominance data. Pixels of converted chrominance components are compared, difference values between the compared pixels are obtained, and representative pixel values are determined by comparing the obtained difference value and an allowable error. The representative pixel values are encoded. A data compression ratio can be improved by discrete-cosine-transforming only portions of an image having different colors from a background color.