C# Bitmap in Graustufen umwandeln und dabei zusehen klappt nicht

        private Bitmap toGreyscale(Bitmap source)
        {
            Bitmap outputImage = new Bitmap(source);

            for (int y = 0; y < source.Height; y++)
            {
                for (int x = 0; x < source.Width; x++)
                {
                    int r, g, b;

                    r = source.GetPixel(x, y).R;
                    g = source.GetPixel(x, y).G;
                    b = source.GetPixel(x, y).B;

                    int greyscale = (r + g + b) / 3;

                    Color greyscalecolor = new Color();
                    greyscalecolor = Color.FromArgb(greyscale, greyscale, greyscale);

                    outputImage.SetPixel(x, y, greyscalecolor);
                }
            }
            return outputImage;
        }

        private void button1_Click(object sender, EventArgs e)
        {
            outputImage = new Bitmap(bmp);
            x = 0;
            y = 0;

            t1.Start();
        }

        int x, y;
        int r, g, b;
        int greyscale;
        Color greyscalecolor = new Color();

        void t1_Tick(object sender, EventArgs e)
        {    
            if (x < bmp.Width)
            {
                r = bmp.GetPixel(x, y).R;
                g = bmp.GetPixel(x, y).G;
                b = bmp.GetPixel(x, y).B;

                greyscale = (r + g + b) / 3;

                greyscalecolor = Color.FromArgb(greyscale, greyscale, greyscale);

                outputImage.SetPixel(x, y, greyscalecolor);
                pictureBox2.Image = outputImage;
                pictureBox2.Refresh();
                
                x++;

                if (x >= bmp.Width)
                {
                    y++;
                    x = 0;
                }
            }
        }

int count = y + (outputImage.Height/32);

BitmapData bmpData;
byte* pixel;
int stride;
byte gP,bP;
bmpData = img.LockBits(new Rectangle(0, 0, img.Width, img.Height), System.Drawing.Imaging.ImageLockMode.ReadWrite, System.Drawing.Imaging.PixelFormat.Format24bppRgb);
pixel = (byte*)bmpData.Scan0;
stride = bmpData.Stride - bmpData.Width * 3;
byte* blue, green, red;
byte newColor;
int w = img.Width, h = img.Height;
{
    for (int k = 0; k < h; k++)
    {
        for (int l = 0; l < w; l++)
        {
            blue = pixel;
            pixel++;
            green = pixel;
            pixel++;
            red = pixel;
            pixel++;
            newColor =(byte) ((*red + *green + *blue)/3);
            *red = newColor;
            *green = newColor;
            *blue = newColor;
        }
        pixel += stride;
    }
}
img.UnlockBits(bmpData);

this.Invoke((Action)delegate
{
   ... //dein Code
});