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, for generating a plurality of gamma parameter data, wherein the cumulative pixel numbers are not larger than a total number of the original pixels corresponding to each of the gray levels and the expanding pixels; adjusting the gray levels of the original pixels according to correlations between the gray levels and the total number of the cumulative pixels; and wherein the step of adding a plurality of expanding pixels comprising: adding a portion of the expanding pixels averagely corresponding to each gray level.