Magical Portraits are a mystery, the Real Ones, not the ones made by painters, but the ones who appear just because, when a wall was empty and now is not, when a portrait with Trolls dancing appears in a long corridor with Vanishing doors.
Everybody knows that magical buildings have magical portraits and the more powerful the building is, the more portraits it has, see Hogwarts for example, full off magical paintings but only a small percentage of them are made on propose, the other ones, the Real Ones, well they just appear one day.
Because of that, no one notices when a small portrait forms in the long corridors of Hogwarts, a painting of a wild meadow. In the meadow a tree, an oak tree is painted and in is roots sleeping, is a small boy, black curly hair and a lightning scar in his brow.