Please tell me a faster way to extract a image from parent image

public static Bitmap GetDifferences(this Image img1, Image img2)
{
    Bitmap parentImage = (Bitmap)img1.GetGrayScaleVersion();
    Bitmap comparer = (Bitmap)img2.GetGrayScaleVersion();
    Bitmap difference = (Bitmap)img2;

    difference.MakeTransparent(Color.Violet);
    for (int y = 0; y < img1.Height; y++)
    {
        for (int x = 0; x < img1.Width; x++)
        {
            if ((parentImage.GetPixel(x, y).R - comparer.GetPixel(x, y).R) == 0)
            {
                difference.SetPixel(x, y, Color.Transparent);
            }
        }
    }

    return difference;
}

public static Image GetGrayScaleVersion(this Image original)
{
    //http://www.switchonthecode.com/tutorials/csharp-tutorial-convert-a-color-image-to-grayscale
    //create a blank bitmap the same size as original
    Bitmap newBitmap = new Bitmap(original.Width, original.Height);

    //get a graphics object from the new image
    Graphics g = Graphics.FromImage(newBitmap);

    //create some image attributes
    ImageAttributes attributes = new ImageAttributes();

    //set the color matrix attribute
    attributes.SetColorMatrix(ColorMatrix);

    //draw the original image on the new image
    //using the grayscale color matrix
    g.DrawImage(original, new Rectangle(0, 0, original.Width, original.Height),
        0, 0, original.Width, original.Height, GraphicsUnit.Pixel, attributes);

    //dispose the Graphics object
    g.Dispose();

    return newBitmap;

}