An apparatus and a method for encoding and/or decoding an image containing a gray alpha channel image. The apparatus for encoding an image includes a block data reception unit receiving image data of a block currently being input to the apparatus and classifies the current block either as a foreground image portion or as a background image portion according to values of gray alpha components in the current block; a foreground image encoding unit sequentially encoding the gray alpha components and brightness and hue components of the current block if the current block is classified as the foreground image portion; and a background image encoding unit encoding the gray alpha components of the current block if the current block is classified as the background image portion. The apparatus for decoding an image includes a bitstream interpretation unit interpreting the bitstream in units of predetermined blocks and classifies a current block obtained as one of the interpretation results either as a foreground image portion or as a background image portion; a foreground image decoding unit generating a restored gray alpha channel image and a restored brightness and hue image by sequentially decoding gray alpha components and brightness and hue components of the current block if the current block is classified as the foreground image portion; and a background image decoding unit generating a restored gray alpha channel image by decoding the gray alpha components of the current block if the current block is classified as the background image portion.