A method and apparatus for encoding an image, and a method and apparatus for decoding an image. The method of encoding an image includes: classifying pixels forming an input image into a first pixel group in which bit depth of original pixel values is maintained, and a second pixel group in which bit depth of original pixel values is changed; generating a changed input image by changing the bit depth of pixel values of pixels in the second pixel group; and performing motion prediction and compensation on the changed input image.