Welcome to the Doge's Palace. The Doge was the elected leader of Venice. However, unlike most modern democracies, not everyone could vote. Only the nobles in power had any say in the "Republic" of Venice.
We walk around the inside of the palace, marveling at the beautiful artwork on display. This is truly the Renassiance, a time where the arts and music became essential to Italian culture. Langdon suggests looking for anything peculiar. Perhaps one of the Doges was in on this secret society, and if so, they might have their own hidden passageway hidden in their gallery. Sure enough, we find a number of potential levers near paintings, but none work except one.
Find the Painting hidden on the Website. This might be a challenge... scan each page closely.