A picture is a rectangle made of n x m picture elements (pixels). Computers represent each pixel as a string of bits that describes the pixel color. A string of n bits can describe 2n colors. Thus, the size of a 3 x 3 black-and-white picture = 3 x 3 x 1 = 9 bits. The size of a 3 x 3 gray (black, white, and 254 levels of gray) image = 3 x 3 x 8 = 72 bits. The size of a 3 x 3 16-color image = 3 x 3 x 4 = 36 bits. The size of a 800 x 600 256-color image = 800 x 600 x 8 = 3840000 bits. Methods as GIF and JPG have been developed so as to compress images so that they are represented using a fewer number of bits.
