public static unsafe byte[] ComputeBitmapHash(this Bitmap bitmap)
{
    var bmpData = bitmap.LockBits(new Rectangle(0, 0, bitmap.Width, bitmap.Height), ImageLockMode.ReadOnly, bitmap.PixelFormat);
    try
    {
        var size = Math.Abs(bmpData.Stride) * bitmap.Height;
        using (var bitmapDataStream = new UnmanagedMemoryStream((byte*)bmpData.Scan0, size))
            return bitmapDataStream.ComputeMD5Hash();
    }
    finally
    {
        bitmap.UnlockBits(bmpData);
    }
}