Pics or it didn't happen is used when someone makes a very far fetched statement about something. See example.
Teenager1: So brah ask me what I did over the weekend
Teenager2: ok....what did you do over the weekend?
Teenager1: I went over to this girls house and I fucked her and then her mom walked in on us and we had a threesome.
Teenager2: Pics or it didn't happen looser!