I am pretty sure if a user posts one of his photos to a forum (like Quora), he can only send a DMCA take down notice. Only if the website owner themselves post the image can they make the copyright claim with damages.
If it were just like you described, then nobody would ever be liable for copyright damages, because they'd just say "oh, my brother owns the site, I just posted, so you can't sue either of us."
And even then, the MPAA, despite it's considerable legal power and ability to get laws altered, still has a difficult time getting damages out of people. "In 2011, for example, the producers for Hurt Locker sued almost 25,000 BitTorrent users—and almost all the claims were voluntarily dismissed by the studio, because it was taking too long to track down all of the defendants via their IP addresses."[1]. That's not difficult for the reasons I said, but it's still difficult, and that's talking about a violently litigious entity with very deep pockets. A regular company or a freelancer is going to have an even more difficult time.
[1]http://www.motherjones.com/politics/2014/02/bittorrent-illeg...
>Members of the general public don't routinely pay for photos in a forum post
Just because lots of people do it doesn't make it right. Rehosting someone else's photo is a violation of their copyright, and should require that you buy a license for the photo. If it's not for sale, then you can't do it, period.