The People in Paintings Are Alive
In the Harry Potter universe, the people in photographs and paintings can move and talk. But it's not like they're just video clips of the person -- hell, we could do that. You actually see characters interacting with the "people" in the pictures, particularly in the portraits that hang all over Hogwarts School -- several major plot points revolve around it. The people in the paintings are able to talk, think and even travel to other paintings. They relay information that no one else has.
"I am innocent but love screaming like a madman."
And as such, some portrait subjects serve as security guards for locked rooms, while others are used as errand boys to deliver urgent messages (they can travel to any room that also has a painting). And, even weirder, these aren't entirely fictional creations dreamed up by some artist, like if somebody made a Mickey Mouse with computerized AI or something -- these are paintings of real people, long-dead former headmasters and such. They possess all of the personality and (presumably) memories of their living selves.
Dumbledore is asleep all the time? Convenient, Rowling. Very convenient.
The Horrific Implications:
Apparently, in the wizard world, if you paint a picture of a dead person, presumably with some magic paint or the right enchantment, their memories and the essence of their personality will be on canvas. Forever. For example, in Harry Potter and the Deathly Hallows, former Slytherin headmaster Phineas Nigellus Black is forced to take orders from his philosophical enemy and student Hermione Granger -- and he is pissed off about it.
Who cares, right? Well, what's stopping that situation from being reversed -- if, after Hermione dies, she gets painted into a portrait and is forced to help some future generation of evil wizard Nazis?
Especially if getting attacked and slashed up is just considered property damage.