1. A method of encoding an image, the method comprising: generating a bitmap table by mapping a quantized image, that is quantized according to a first bit-depth, to a map table, and setting a bitmap index corresponding to each pixel location of the quantized image with reference to the bitmap table; allocating the bitmap index having a second bit-depth to each pixel based on representative level values, the second bit-depth being smaller than the first bit-depth; determining a filter index corresponding to a pre-defined filter type for each pixel of the image according to local image characteristics by analyzing local characteristics of the image; and generating bitmap data for each pixel of the image by adding the filter index to the bitmap index.