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.