1. An image enhancement method, comprising the steps of: acquiring a digital image; selecting a specific pixel from a plurality of pixels of the digital image; selecting some of the pixels surrounding the specific pixel to constitute a specific block; calculating an average of values of the pixels of the specific block; providing a plurality of curve functions; substituting the average of the values of the pixels of the specific block into the curve functions to obtain a plurality of output brightness values; and finding one of the output brightness values, which is closest to a specific output brightness value of a specific brightness value, to find out a specific curve function corresponding to the specific output brightness value.