Dumbledore didn’t get into magic for afterlife ghost. He didn’t like it. But in books, he was made a memory or something like that in his own portrait. That is it is not exactly Dumbledore trapped in portrait but his memoir. And for the answer Dumbledore didn’t like the purpose for being ghost, but he was as a memoir in Hogwarts. More description can be found in the last pages of Deathly Hallows book and in Cursed Child book.

Это вроде пересказ интервью Роулинг