Single people that post pics of themselves with an ex on dating apps are too lazy to take new photos for their profile. It’s much easier to post old pics that they think they look good in and post them to a dating profile, than go out and take fresh pics.
That said, there are guys on dating apps who purposely post pics of themselves next to an ex. He thinks that by doing this, he shows the girls that see his profile that he has had a history of dating girls and having girlfriends.
He believes that by having pics with a girl next to him, he is indicating that girls are attracted to him, and that he has experience dating girls.
He thinks that this will make him more attractive to girls that see his dating profile because it gives the impression that he is already desirable to women.