struct BitmapDataAccessor
{
    private readonly byte[] data;
    private readonly int[] rowStarts;
    public readonly int Height;
    public readonly int Width;

    public BitmapDataAccessor(byte[] data, int width, int height)
    {
        this.data = data;
        this.Height = height;
        this.Width = width;
        rowStarts = new int[height];
        for (int y = 0; y < Height; y++)
            rowStarts[y] = y * width;
    }

    public byte this[int x, int y, int color] // Maybe use an enum with Red = 0, Green = 1, and Blue = 2 members?
    {
        get { return data[(rowStarts[y] + x) * 3 + color]; }
        set { data[(rowStarts[y] + x) * 3 + color] = value; }
    }

    public byte[] Data
    {
        get { return data; }
    }
}

    public static byte[, ,] Bitmap2Byte(Bitmap obraz)
    {
        int h = obraz.Height;
        int w = obraz.Width;

        byte[, ,] wynik = new byte[w, h, 3];

        BitmapData bd = obraz.LockBits(new Rectangle(0, 0, w, h), ImageLockMode.ReadOnly, PixelFormat.Format24bppRgb);

        int bytes  = Math.Abs(bd.Stride) * h;
        byte[] rgbValues = new byte[bytes];
        IntPtr ptr = bd.Scan0;
        System.Runtime.InteropServices.Marshal.Copy(ptr, rgbValues, 0, bytes);

        BitmapDataAccessor bda = new BitmapDataAccessor(rgbValues, w, h);

        for (int i = 0; i < h; i++)
        {
            for (int j = 0; j < w; j++)
            {
                wynik[j, i, 0] = bda[j, i, 2];
                wynik[j, i, 1] = bda[j, i, 1];
                wynik[j, i, 2] = bda[j, i, 0];
            }
        }

        obraz.UnlockBits(bd);
        return wynik;
    }

    public static Bitmap Byte2Bitmap(byte[, ,] tablica)
    {
        if (tablica.GetLength(2) != 3)
        {
            throw new NieprawidlowyWymiarTablicyException();
        }

        int w = tablica.GetLength(0);
        int h = tablica.GetLength(1);

        Bitmap obraz = new Bitmap(w, h, PixelFormat.Format24bppRgb);

        for (int i = 0; i < w; i++)
        {
            for (int j = 0; j < h; j++)
            {
                Color kol = Color.FromArgb(tablica[i, j, 0], tablica[i, j, 1], tablica[i, j, 2]);
                obraz.SetPixel(i, j, kol);
            }
        }

        return obraz;
    }

Now, if I do:

    private void btnLoad_Click(object sender, EventArgs e)
    {
        if (dgOpenFile.ShowDialog() == DialogResult.OK)
        {
            try
            {
                Bitmap img = new Bitmap(dgOpenFile.FileName);

                byte[, ,] tab = Grafika.Bitmap2Byte(img);

                picture.Image = Grafika.Byte2Bitmap(tab);
                picture.Size = img.Size;


            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
            }
        }
    }

Most of pictures are handled correctly butsome not. Example of picture that doesn't work:

It produce following result (this is only fragment of picture) :

Why is that?