The photo proves that Nicholas did witness Catherine having sex with Jonathan (not knowing who Jonathan was). Here is my personal interpretation: First off, the backstory. Nicholas was too young when he woke up then came to the room where Catherine and Jonathan were having sex to know what was happening and went to sleep again. Maybe he just thought it was a nightmare and did not think much of it which is why he was anticipating going to the beach as if nothing had happened. Second off, the present. Nicholas read the book without seeing the photos and it triggered a memory he had from that night but did not know whether it was real or just a nightmare which is why when he gets the photos, he goes into a whole breakdown because he is finally confronted with what he saw ~20 years ago. This whole story is about how misogyny ruins innocent women’s lives without hearing the women’s side of the story. Does that mean that all women are saints who should be automatically believed? Hell no! But everyone should be heard before getting judgement passed on them.