private static void Main(string[] args)
{
    path = @"D:\Universität\Visual Studio Projects\Console Apps\PictureInvert\PictureInvert\ae86.png";
    Init();
}

private static void Init()
{
    // Initialize Bitmap into variable bmp
    bmp = new Bitmap(path);

    Invert();

    // Save the Bitmap
    Save();
}

private static void Save()
{
    string[] str = path.Split(".");
    bmp.Save(str[0] + "_INV." + str[1]);
}

private static void Invert()
{
    int w = bmp.Width;
    int h = bmp.Height;

    Color color;
    byte r, g, b;

    // Invert matrix
    for (int y = 0; y < h; y++)
    {
        for (int x = 0; x < w; x++)
        {
            color = bmp.GetPixel(x, y);

            r = color.R;
            g = color.G;
            b = color.B;

            bmp.SetPixel(x, y, Color.FromArgb(255 - r, 255 - g, 255 - b));
        }
    }
}