1. An image processing method, applicable to a pointing device, wherein the pointing device generates a displacement signal by moving relative to a working surface and detecting an image signal of a part of the working surface, and the method comprises: successively obtaining a first original image and a second original image; uniformly dividing the first original image into a plurality of first blocks and dividing the second original image into a plurality of second blocks; adjusting a block brightness value of each of the first blocks and the second blocks; generating a complexity signal representing a plurality of gradient values of each of the first blocks and the second blocks according to the corresponding block brightness value; adjusting each of the first blocks according to the adjusted corresponding block brightness value and the corresponding complexity signal of the first original image, so as to generate a first adjusted image corresponding to the first original image; adjusting each of the second blocks according to the adjusted corresponding block brightness value and the corresponding complexity signal of the second original image, so as to generate a second adjusted image corresponding to the second original image; and executing a motion estimation procedure according to the first adjusted image and the second adjusted image, so as to generate the displacement signal.