I (out of the blue) received a photo of a guy’s dick from his friend. Yes, I slept with him a while ago but why would I get a photo from his female friend? Better question, what’s the Coquettiquette?
Holy shit. Coquettiquette? You are awesome for coming up with that.
As for your question, it’s probably one of two things. She’s either marking territory or just fucking around, neither of which require a response. Best thing to do is just continue not giving a shit.
Save the pic, though. It might come in handy later.