So he is under arrest, in a squad car or booking room, and he sends an incriminating photo to a group list? He’s a punk but I don’t think he’s that stupid.
I suspect he sent the picture during the rape. "Haha look at this passed out girl. I moved/removed her top, and she didn't wake up!" Or some such nonsense. And then he decided to do more. It's a shame whatever app he used (Snapchat?) didn't save the message.
He sends it out after he disrobes her, prior of fingering and shoving pine needles into her vag. Swim buddy receives it and retweets (which is the source of the image they have)a little later - after the perp has been arrested (he never thought he’d get caught)