1. An image processing device, comprising: an image determining unit configured to determine a kind of an input image as one of a moving picture, a photograph image, and a user interface image; a legibility improving unit configured to generate a legibility improved image by extracting specific pixels from pixels included in the input image according to the kind of the input image that is determined by the image determining unit, and wherein the legibility improving unit is further configured to modify gray scale values of the extracted specific pixels; an illuminance sensor for sensing external illuminance; and a brightness improving unit for correcting a gamma curve according to the external illuminance sensed by the illuminance sensor.