As well, the article mentioned that this is the first photo of him with some of his known gang members. The man to his left is quite clear, and therefore if they can identify him as an associate of Billy The Kid, perhaps that gave them more reason to suspect it was Billy.
As for the associate, my sister thought it looked liked two boys in the midst of arguing over rules and cheating in the way that older and younger brothers do...
http://www.truewestmagazine.com/billy-the-kid-experts-weigh-...
Apparently four people in the photo have also been identified as those known to associate with Billy the Kid, including Sally Chisum. Her diary seems to confirm this event taking place - which was a cattle drive and the wedding of Charlie Bowdre (on the horse).
So details of the verification process are being withheld in order the make the documentary more appealing.