Like some of the portraits in RDR2, I assume that he is just there for decor. He could related to the foundation of Blackwater or that bank.

So, it is not a real guy?

Of course he ain't a real guy, but he could be based off of someone or is just there for decor like the other fellow said. Like how the states in the game(s) are based off the real American frontier, how Lemoyne is like Louisiana, or New Hanover is like Kansas, Missouri, Etc. (Midwest)

While its very possible it's a custom portrait made specifically for the game, there are some portraits in RDR2 and some of the bills in RDR1 depict real photos of real people, if there was a higher quality photo of the image someone could reverse image search it to see if it is unique to the game or not