Apparently while Erik paid a little visit to Raoul's quarters, he discovered a little nugget of gold among his dusty belongings... After smuggling the artifact and returning to the Opera, Erik couldn't resist revealing the treasure to Christine (who soon after seeing the painting burst into a fit of giggles). Raoul, infuriated, wished his father never hired a painter all those years ago...
And WHAT makes the small portrait so humorous, you might ask?
Well... I wish I could fully explain it... but it all probably lies in the fact that in the painting, Raoul is a boy; not much older than eight, wearing a sailor's outfit -- one he most likely wore on one of his first childhood voyages with his older brother. Perhaps Erik and Christine began choking in laughter because they observed a pattern -- over the years, The Vicompte's hair-style hadn't changed drastically.
Either way, we may blame Erik for his cruel joke. At least no one got hurt...
But this is kinda funny.