1. A circuit for automatically detecting image flicker, the circuit comprising: an image sensor for capturing an image frame according to an exposure setting; an average unit coupled to the image sensor for generating a luminance average value corresponding to each pixel row according to a luminance sum of predetermined pixels of the pixel row of the image frame, wherein the predetermined pixels of the pixel row are green pixels of the pixel row; a difference generation unit coupled to the average unit for generating a difference corresponding to each corresponding pixel rows of two consecutive image frames captured by the image sensor according to a luminance average value of a pixel row of a first image frame of the two consecutive image frames and a luminance average value of a corresponding pixel row of a second image frame of the two consecutive image frames; a transform unit for performing a discrete Fourier transform on the difference corresponding to the corresponding pixel rows of the two consecutive image frames to generate a transformation result; and a flicker detection unit coupled to the transform unit for determining whether flicker is shown in the two consecutive image frames to generate a detection result according to the transformation result.