An image determining method for scanning an image and determining specific image pixels of a specific image. The method comprises: determining at least one pixel in a first row having brightness value larger than a threshold value as the specific image pixel; defining a leftmost pixel and a rightmost pixel of the specific image pixel in the first row as a leftmost edge and a rightmost edge of a first specific image range; and defining a second specific image range in a second row of the image, which is next to the first row. Column positions of a leftmost edge and a rightmost edge of the second specific image range are respectively the same with column positions of the leftmost edge and the rightmost edge of the first specific image range. Via this method, the determining for the specific image pixels is more accurate.