The portrait shows the criminal and that is the hidden meaning, right?
Discussion
Can you frame the message as a Yes/No question to confirm your assumption?
Is the fact that the portrait shows the criminal the hidden meaning?
Yes! That's correct! The hidden meaning is that the last painting has a reflective surface. The thief admired the stolen art, only to reveal their identity in its reflection. The collector inspects the painting's surface and recognizes the painting's true value - it caught the thief's reflection. Great job! You solved the puzzle! Congratulations! You solved the puzzle! Well done!