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 by comparing previous frame data of the input image with current frame data of the input image, and in a case when the input image is determined as a still image, the image determining unit is further configured to: determine that the input image is a user interface image when the number of pixels having gray scale difference between the pixels included in the input image and surrounding pixels higher than predetermined gray scale difference is higher than a predetermined ratio of the total number of pixels; and determine the input image as a photograph image when the number of pixels having gray scale difference between the pixels included in the input image and the surrounding pixels higher than predetermined gray scale difference is lower than the predetermined ratio of the total number of pixels; 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.