1. An image preprocessing method for processing an input image having a plurality of pixels arranged in an array form to increase the accuracy of a subsequent face detection, a pixel at column m and row n of the input image having an image value f(m,n), the preprocessing method comprising the steps of: (A) for each pixel (m,n) of the input image, determining a sum TOP a (m,n) of PN pixels on row n−a closest to the pixel (m,n), and a sum BOT a (m,n) of the PN pixels on row n&plus;a closest to the pixel (m,n), wherein PN is an integer of at least 2, and the superscript ‘a’ is a positive integer; (B) determining a difference DT(m,n) between the pixel (m,n) and the determined pixels on the row n−a by DT(m,n)&equals;PN*f(m,n)−TOP a (m,n), and a difference DB(m,n) between the pixel (m,n) and the determined pixels on the row n&plus;a by DB(m,n)&equals;PN*f(m,n)−BOT a (m,n); (C) if &verbar;DT(m,n)&verbar;>&verbar;DB(m,n)&verbar;, providing an image difference DIF(m,n)&equals;DT(m,n) otherwise, providing an image difference DIF(m,n)&equals;DB(m,n); and (D) determining a temporary value T(m,n) of the pixel (m,n) by T(m,n)&equals;(&agr;*f(m,n)&plus;DIF(m,n))/&bgr; for use as a new image value f′(m,n) of the pixel, where &agr; is a positive enhancement factor and &bgr; is a positive division factor to make the value of T(m,n) range between 0 and a maximum gray-level.