Between Cloud's high Knowledge (Arcana) check and Luigi's Analyze Dwoemer, they know there is no additional spell cast on the painting protecting.
It would be highly unlikely that there is something built into the item that prevents such an act.
That leaves two potential bad circumstances, relating to Malchia's question.
1 - Someone could be alerted when it is destroyed (as per a deity and its artifact OR a spell cast on the someone that is set to go off when an event happens)
2 - The destruction of a permanent portal causes an explosion or other calamity.