When you click on the paintings, the grare will flash multiple times, if you hold down the knocker it counts as... a dash let's say (this - ) and if you click and don't hold it would be like a dot (this . ). it's basically morse code lol. click on a painting and the watch the glare. it will flash, then copy it's actions to the doorknob. (hope this helps)
ex:
flaaaaash flash flash flaaaaash = KNOOOOCK knock knock KNOOOCK