"Everard and Dilys were two of Hogwarts's most celebrated Heads. Their renown is such that both have portraits hanging in other important wizarding institutions. As they are free to move between their own portraits, they can tell us what may be happening elsewhere…"
-- Dumbledore (OP22)
Dilys Derwent was a celebrated Headmistress of Hogwarts and a famous Healer. She worked at St Mungo’s Hospital from 1722 until becoming Headmistress of Hogwarts in 1741. She remained Head of the school until 1768 (OP22). Copies of Derwent’s portrait hang not only in Dumbledore’s office but in the main admitting area for St. Mungo’s, a fact which come in handy when Dumbledore wants first-hand information about patients being brought into the hospital. It was Derwent who, by moving to her painting in St Mungo’s, was able to let Dumbledore know that Arthur Weasley was still alive when brought into the hospital after being attacked by Nagini (OP22). When Harry visited the Head’s office after his victory over Voldemort, he was greeted by various expressions of joy from the portraits there. Derwent was weeping unashamedly (DH36).