Taken from the HP4GU FAQ's guidelines.

HPSS - an illustration from Harry Potter and the Sorceror's Stone. Book 1.
  Sorry, I didn't have the Philsopher's Stone version, and it's a bit of a pain to include the PS/ part of the naming convention in a filename.

HPCOS - an illustration from Harry Potter and the Chamber of Secrets. Book 2.

HPPOA - an illustration from Harry Potter and the Prisoner of Azkaban. Book 3.

HPGOF - an illustration from Harry Potter and the Goblet of Fire. Book 4.

HPFB - an illustration from Fantastic Beasts and Where to Find Them.

HPQTA - an illustration from Quidditch Through the Ages.