1. An method for adjusting image, applied to an image output device, the method comprising the following steps of: obtaining a current image frame comprising a plurality of pixels; analyzing intensities of input image signal values of the pixels; defining each of the pixels having the input image signal value being smaller than an critical value as a dark pixel, and counting a number of the dark pixels to obtain a ratio of the dark pixels among all the pixels to serve as a dark pixel ratio; determining an image adjusting curve equation according to the dark pixel ratio; and substituting the input image signal value of each of the pixels to the image adjusting curve equation, obtaining an output image signal value of each of the pixels, and combining all the output image signal values to obtain a final output image frame.