NdA I´m Spanish so tell me if my English sucks, please.
J.K.Rowling owns the Potterverse.
The portrait
After the war, when Hogwarts was rebuilt and its doors were opened again, Draco Malfoy, who had yet to finish his NEWTs, hung a portrait in the Slytherin Common Room. He wasn't alone. When his classmates noticed what he was doing, they approached to him, staring at him with serious eyes which understood everything. Professor Slughorn, who was then the Head of Slytherin, also went to see what was happening. He opened his mouth as if he wanted to protest, but the stares he received, hard stares which showed no mercy, made him close it and nod in agreement.
Only the Slytherin knew the existence of this portrait. When Draco's year left the school, there were still a lot of people who knew its meaning, because they had passed through the same thing. And they explained it to the new students, so they could know it too. Some years later, there wasn't any student left in Hogwarts who had known him, but the portrait remained there and its painful lesson was delivered again and again. Slughorn never dared to remove it.
Year after year, like and old ritual, the first year students are driven to the portrait and they listen to its story. It is hung upside down as a symbol of dishonour and it shows a man with dark hair, pale skin and hooked nose. It's Severus Snape, they explain, the traitor. The Head of Slytherin who bertrayed all his students. And they tell the story. He, who had to take care of all these children, guide them while they were in Hogwarts, only used them to pretend that he was still loyal to Voldemort. He incited them to despise Harry Potter and other Gryffindors, he helped to put them against the Side that he actually supported. When he became the Headmaster, supposedly following Voldemort's orders, he allowed the Slytherin students to torture the rebel students as punishment. And then he left them alone with no worries about their futures, abandoning them to their own luck.
The new students look with scorn at the man in the portrait. They are Slytherin and they understand it. The Head of Slytherin who didn't protect his people.
Severus Snape, the traitor.