Quick and Dirty Output of WriteableBitmap as TGA Image

public static class TgaWriter {
    public static void Write(WriteableBitmap bitmap, Stream output) {
        int width = bitmap.PixelWidth;
        int height = bitmap.PixelHeight;
        int[] pixels = bitmap.Pixels;
        byte[] pixelsArr = new byte[pixels.Length * 4];

        int offsetSource = 0;
        int width4 = width * 4;
        int width8 = width * 8;
        int offsetDest = (height - 1) * width4;
        for (int y = 0; y < height; y++) {
            for (int x = 0; x < width; x++) {
                int value = pixels[offsetSource];
                pixelsArr[offsetDest] = (byte)(value & 255); // b
                pixelsArr[offsetDest + 1] = (byte)((value >> 8) & 255); // g
                pixelsArr[offsetDest + 2] = (byte)((value >> 16) & 255); // r
                pixelsArr[offsetDest + 3] = (byte)(value >> 24); // a

                offsetSource++;
                offsetDest += 4;
            }
            offsetDest -= width8;
        }

        byte[] header = new byte[] {
            0, // ID length
            0, // no color map
            2, // uncompressed, true color
            0, 0, 0, 0,
            0,
            0, 0, 0, 0, // x and y origin
            (byte)(width & 0x00FF),
            (byte)((width & 0xFF00) >> 8),
            (byte)(height & 0x00FF),
            (byte)((height & 0xFF00) >> 8),
            32, // 32 bit bitmap
            0 };

        using (BinaryWriter writer = new BinaryWriter(output)) {
            writer.Write(header);
            writer.Write(pixelsArr);
        }
    }
}

private void btnSave_Click(object sender, RoutedEventArgs e) {
    WriteableBitmap _screen;
    // ... do something with the WriteableBitmap here...

    SaveFileDialog dialog = new SaveFileDialog();
    dialog.DefaultExt = ".tga";
    dialog.Filter = "TGA Image (*.tga)|*.tga";
    dialog.FilterIndex = 1;

    if (dialog.ShowDialog() != true) return;
    using (Stream fileStream = dialog.OpenFile()) {
        TgaWriter.Write(_screen, fileStream);
        fileStream.Close();
    }
}