I see what you're saying. Let's see. We're looking at probably CR 2-3, 4 tops, so DR X/magic will be hard to overcome, if people even think to attack the painting (esp if their friend is inside!). Why don't we go with a mix of things like DR 5/magic, hardness 5, and 10 hp? We can also list a knowledge (arcana) (or spellcraft?) check needed to learn that destroying the painting returns the epadrazzil to its home plane and release any entrapped characters. DC of 20 or something, once you've already identified the epadrazzil via a knowledge check.