Disclaimer: I don't own Naruto or Harry Potter. I'm neither japanese or brithish thank you very much.
Nowadays you could be hired as the headmaster in some school's even though you had not worked in the school as a teacher (sometimes you did not even need to have worked as a teacher, ever). The only things needed are the right contacts, and enough of credentials. Well, in some school's in the muggle-world, in the wizarding-world you would have to work as a teacher in the school, and having some money in the pocket you liked to give away... Well, that helped more then often.
In Hogwarts history it has not been many Head masters from Slytherin, because the compulsory "need to work as a teacher to become one". Because of the Slytherin-ambition, few Slytherin decides to work as a teacher, you see. One of those few, few Head Masters who was from Slytherin (and had gotten his painting stuck on the Head Master office-wall), was Phineas Nigellus Black.
Because Pihneas was the only Slytherin on the wall of buffoon (which he called it in his mind), he recognised that Harry Potter lied about what happened in the chamber of secrets. The other paintings was to idiotic (or asleep) to recognise the telltale signs of a lie, but not Phineas, he could easily see it. Why the brat lied, he did not know, if the boy had killed the basilisk he would probably brag as the brat he was, if somebody else then the idiot teacher (who had been taken away, because his status as passed out) had killed the basilisk the brat would have stolen the glory as the brat he was, and if the idiot teacher had killed the basilisk in some other way (transfigure a stone to a stone, and luckly missing being bitten), the boy had no reason to lie about it.
Phineas glanced under half-closed eyes to Dumbledore. The old sack of wrinkles and bones did not see the lie, it was obvius to Phineas Slytherin-eyes, and seemed belive the boy. Idiot.
Phineas shrugged. It was not his problem if Dumbledore decided to be an idiot and believe the brat. The muggle-loving twit should find out, and deal with, the lie himself. Not his problem at all.
And with that thought, phineas fell asleep, dreaming about his glory-days as Head Master.
You should not expect a fast updating on the story even if the hiatus is currently lifted (thanks to that school's out here in my town). I know the drabble is a little boring, but please review (maybe the update get faster...).