Actually, I think the dress Angelique is wearing in the portrait is the
brownish dress with lace that she wore when she did the "house of cards"
fire spell during Vicki's exorcism. So she WAS Mrs. Barnabas Collins
when she wore that dress. As for the portrait, as others have said, it
did first appear when Bathia was trying to lift Barnabas' curse in the
Old House.