In the era of smartphones and showing-off on social media, to believe anything far-fetched, photographic proof is expected. The above phrase simply says that if one doesn't have proof, the claimed thing doesn't exist.
Also see : "Pics or it didn't happen"
A : Hey Bro, I just got a Harry Potter 1st edition book, autographed by J K Rowling herself !!
B : Pics or it doesn't exist
Ruler of Lake Titicaca. Rumored to have a bunghole that gets very angry if it does not receive toilet paper. Cornholio the Great is often seen walking around with his shirt over his head and his hands in the air, chanting songs about his power, and his bunghole.
"I am Cornholio! You do not want to face the wrath of my bunghole, for I need TP!"
Butthead: Shut up, Beavis! (uh huh huh huh)
Beavis: Um, okay. (heh heh heh heh).
Boys who enjoy yaoi (a genre in Japan that contains sexual and/or romantic relations between two men); literally translates to "rotten boy"; corresponding female : fujoshi