An apparatus and method for improving qualities of motion and still images to be output in a mobile communication terminal. An error-reflected-value table is provided to reduce a computation amount while applying an error diffusion process. The table includes quantization-error-reflected values pre-computed according to maximum values of quantization errors and error-reflected ratios of neighboring pixels. When quantization errors of a specific pixel of an original image selected in a process procedure are computed, error-reflected values of neighboring pixels are determined. Pixel-by-pixel error values are stored by accumulating error-reflected values related to pixels. When error-reflected values of neighboring pixels around all the pixels of the original mage are completely computed, an image in which quantization errors are diffused is generated when the error-reflected values are added on a pixel-by-pixel basis. The error-diffused image is stored and displayed in place of the original image when a user makes a request.