1. An image adjustment method comprising: receiving an image data, wherein the image data has a plurality of original pixels, the number of the original pixels is Num 1 , and 2 N−1 ≦Num 1 ≦2 N −1, wherein N and Num 1 are natural numbers; adding a plurality of expanding pixels to the image data, so that the sum of the original pixels and the expanding pixels is equal to 2 N −1; and calculating a plurality of cumulative pixel numbers corresponding to a plurality of gray levels, wherein the cumulative pixel numbers are not larger than the original pixels of each gray level and a total number of the expanding pixels; and adjusting the gray levels of the original pixels according to correlations between the gray levels and the total number of the cumulative pixels.