How would a feller go about copyrighting a pic?
Copy rights exist de facto - that is, unlike a patent, intellectual works (photos, written pieces, artwork, etc.) are protected by the doctrine of copyright, not by a bureaucratic process or sanction by a governmental agency.
There is, however, a doctrine called "fair use," which is an exception an author's right to exclusion of a creative work. This includes things like commentary, news reporting, teaching and scholarship, and others, and also explains why the news can show clips of an NFL game, and you can invite your buddies over to watch, but you can't open a theater and sell tickets to see a replay of the broadcast.
Someone who makes their living by creating, like a photographer, or someone who has created a work that might have commercial value, like a naked photo of a celebrity, obviously has more interest in enforcing their copyrights than the run-of-the-mill shots posted on Facebook. Given how special this photo is, and that it cannot be replicated today, I can understand why it's creator would want credit and perhaps renumeration for it, particularly when a for-profit magazine is using it to make sales.
As for pictures someone here took posted on WildHorses' facebook page, these may fall under the fair use doctrine, but if they don't, the question becomes whether there is financial incentive to enforce the copyright, not whether it exists - it certainly does. If WH is using the photo without permission to sell product, there might be some small incentive, but I doubt it (since they could presumably take a photo of the product on another right themselves). If it bothers you, though, I would think if you contacted them and requested them to take it own or not use it, they could comply.