a method for partially up / downscaling an image encoded on a macroblock basis utilizing a computer , includes : a step of storing the encoded image in a storage device of the computer ; a step of creating map data from bitstream of the encoded image , the map data being utilized to decode at least one macroblock of the encoded image , creating a shrunken image of a predetermined size based on a screen resolution of a display device , and storing the map data and the shrunken image so as to relate the map data and the shrunken image with the encoded image in the storage device ; a step of outputting , to the display device , the shrunken image related with the encoded image to be displayed based on a control request received from an input device ; a step of determining at least one macroblock to be decoded based on a display area of the shrunken image ; a step of partially decoding the encoded image for the determined macroblock using the map data ; and a step of outputting to the display device , the image data of the display area of the partially decoded image .