Book 7 | Ch:18 - The Life and Lies of Albus Dumbledore

SPOILERS:

Hermione gives Harry a copy of The Life and Lies of Albus Dumbledore that she picked up in the Bathilda Bagshot's home, and they learn that the young man in the picture was Gellert Grindelwald. According to Rita Skeeter, he was very close to Albus Dumbledore for a brief period when they were young, at a crucial period in both their lives.