1. An image processing method, used to scale-upscale-up and scale-down an image with the same hardware, wherein the image comprises a plurality of pixels ({a k j , 1□k□M, 1□j□N}) arranged in a plurality of lines (M lines) and in a plurality of columns (N columns) forming a two-dimension array, comprising: providing a line buffer comprising at least 2N storage cells; providing a first variable and a second variable for cooperating with the line buffer to scale-upscale-up and scale-down the image; and practicing a scaling-down procedure when scaling-down the image, wherein the scaling-down procedure comprises: storing the pixels ({a k j , j=1, 2, 3,. .. N}) of the k-th line of the array into the line buffer from the first storage cell to the N-th storage cell in order, respectively; storing the pixel (a k+1 j ) of the (k+1)-th line and the j-th column of the array into the first variable; storing the pixel (a k+1 j+1 ) of the (k+1)-th line and the (j+1)-th column of the array into the second variable; practicing a first algorithm to determine a first new pixel (b k j ) based on the pixels (a k j , a k j+1 , a k+1 j and a k+1 j+1 ) stored in the j-th column, the (j+1)-th column of the line buffer, the first variable and the second variable, respectively, and outputting the first new pixel (b k j ); removing the pixel stored in the first variable to the j-th storage cell of the line buffer; removing the pixel stored in the second variable to the first variable; storing the pixel (a k+1 j+2 ) of the (k+1)-th line and the (j+2)-th column of the array into the second variable; and practicing the first algorithm to determine a second new pixel (b k j+1 ) based on the pixels (a k j+1 , a k j+2 , a k+1 j+1 and a k+1 j+2 ) stored in the (j+1)-th column, the (j+2)-th column of the line buffer, the first variable and the second variable, respectively, and outputting the second new pixel (b k j+1 ).