"You know what to do" -- Aberforth Dumbledore (DH28)
The portrait of Ariana Dumbledore resides over the small fireplace in Aberforth Dumbledore’s sitting room above Hog’s Head. It is a large oil painting of the young blonde girl. Ariana is described as gazing out of the portrait with “a kind of vacant sweetness.” The mirror that once belonged to Sirius sits beneath the portrait on the mantle. The portrait acts as a doorway to the tunnel that leads to the Room of Requirements. Instead of leaving the portrait through the side of her frame, like most people in portraits do, Ariana is able to move down a long tunnel painted behind her. Other people (who are not painted) appear in her portrait when they are walking down the tunnel. Ariana has the ability to communicate messages from Aberforth to the residences of the Room of Requirements.
"When a magical portrait is taken, the witch or wizard artist will naturally use enchantments to ensure that the painting will be able to move in the usual way. The portrait will be able to use some of the subject’s favourite phrases and imitate their general demeanour. Thus, Sir Cadogan’s portrait is forever challenging people to a fight, falling off its horse and behaving in a fairly unbalanced way, which is how the subject appeared to the poor wizard who had to paint him, while the portrait of the Fat Lady continues to indulge her love of good food, drink and tip-top security long after her living model passed away" (PM).