Yes, he really existed, and here's the photographic proof.
As you know by now, Moonlight Graham, dredged out of obscurity by W.P. Kinsella in his novel Shoeless Joe (which was later made into the film Field of Dreams), was a real dude. Kinsella got his biographical timeline right, but Hollywood changed all of Graham's significant dates because...well, I guess they had their reasons.
Graham (now) famously played one defensive inning for the New York Giants and never got to bat. He did, however, have a seven-year minor league career. He spent all or part of four of those years with the Scranton Miners of the Class B New York State League. This postcard was produced his first year with the team, 1905, which was also the same season he had his sip of coffee with the Giants.