1. An apparatus to encode an image containing a gray alpha channel image, the apparatus encodes the image including gray alpha components and brightness and hue components in units of predetermined blocks, the apparatus comprising: a block data reception unit which receives 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 the values of the gray alpha components in the current block; a foreground image encoding unit which sequentially encodes the gray alpha components and the 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 which encodes the gray alpha components of the current block if the current block is classified as the background image portion.