Two Flints:
Probably the library or an Internet search would be the best place to look up a lot of these men. The American Plainsmen Society covers the years 1840 to 1865 -- basically that time frame between the mountain man era and groups that focus on the 1865 and later period. We took the name Plainsmen early on, but include just about any persona from the American frontier period -- Dragoons, 49ers, Texas Rangers, Indian scouts, Crackers, farmers and settlers, townsmen and women, border ruffians, bandits, etc.
There's no doubt the actual Plainsmen period ran later than 1865, and there's more than a few of us who wish we'd run the date up to at least 1870. But the dates we chose fills a niche for historical re-enactment, and that was our original goal.
As far as the photos:
A lot of people say Liver Eating Johnson went West in 1847, but other research claims a post-Civil War date. Based on the information I've seen, I tend to believe the later. I could be wrong. (Wouldn't be the first time.)
I've seen this muleskinner photo before. I don't recall where, but I believe he was identified as an 1850s bullwhacker. His style of whip would support that. Bullwhackers general carried a whip with a longer staff to reach over the tops of the cattle they were walking alongside.
Again, off the top of my head, I don't think Billy Dixon came West until after the Civil War; my memory is that his first Western experience was as a wagon driver hauling trade goods to the Medicine Lodge Treaty Council of '66 or '67. That should be easy to find with a search, and my apologies if I'm wrong.
You'd probably be more aware of when Frank North came West -- unless he was born there. I'm thinking he was, but I don't know what he did prior to becoming commander of the Pawnee Scouts after the Civil War.
I don't know anything about the "Unknown Frontiersman", but my first impression was that he seems a mite "untarnished" to have spend much time outdoors.
Beckwourth could easily be considered a Plainsmen.
I'm not familiar with James Stobie, but he certainly looks the part. If anyone has more information on him, I'd like to see it.
I've not familiar with Hank Greenwood, either, but I'm curious about what appears to be two belts. His other gear would date him after our time frame. Of course that doesn't mean he couldn't have been on the frontier quite a bit earlier.
I like your idea of posting photos or prints from the 1840 to 1865 time frame. A picture adds a level of understanding that a written description can't -- and vice-versa. Keep 'em coming.