I would say that the following are what should be considered spoilers:
-Anything alluding to the meteor strike, but not the comet since it's a recurring thing in the movie that seems insignificant at first.
-Anything explaining how they met in a train. Especially if it mentions the time of the meeting.
-Adult Taki and Mitsuha.
-Anything related to Taki and Mitsuha's efforts to save the town from the meteor strike.
-Anything referencing the where-they-are-now scenes of the survivors near the end.
-Anything alluding that they forgot each other.
I'm not sure about the mountain crater scene, since a picture alone won't spoil what happens there but knowing that they will meet will likely spoil that scene even though the more important part about that scene happens afterwards. So I think this one's a bit borderline.
Probably a bit too specific and cautious, but that's my two cents on the matter.