I've noticed the same technique on dating apps. 9/10 times if the person you're talking to is not real they'll send a 'casual' looking photo after 3-4 back and forth messages, completely unsolicited.
Like the author of this article it seems to be a misguided attempt to prove that they're real somehow and immediately gives away that they're not. When this happens on dating apps I like to give them my Google Voice number and play along for a bit to waste their time. They usually end up pretty angry and it makes me laugh.