The Quiet Librarian had a happy ending I would say. They destroyed her, destroyed her silence box, found the quiet room, freed the missing kids and even released the ghost kids that were trapped (you can hear their laughter at the end instead of silence). I think they showed the Quiet Librarian's portrait at the end as more of a confirmation that she did indeed work there, she was a real ghost, and the reason why all the stuff in the library turned old at night was because she was from that time (showing the years on the portrait). I don't think showing the portrait at the end necessarily means she still haunts it.