Cool thread idea, Billy!
Narrowing it down to one would be REALLY hard.
I think I might go with a shot of Christy Mathewson in pitching motion, during the fifth game of the 1905 World Series. Ideally, the shot would have been taken from directly behind home plate, much like those World Series first pitch shots from the 1950s. So, you'd have Christy facing whomever on the Athletics, complete with a panoramic view of the Polo Grounds right behind him.
Photographer-wise, I can't really think of anyone specifically who would have been there to take such a shot, aside from Conlon. Though, that sort of thing wasn't necessarily in his wheelhouse anyways (or at least from what we know about surviving action shots that he took). I would just want the photograph to be clean and clear. And of course, enough for me to base a painting off of.
Graig