I'm not sure when the picture was put in, but I never noticed it until I did the Marceline to Magic Kingdom tour in November 2019. It's tied in to the group of bust statues that are in the standby queue. Kind of a little murder mystery thing and the picture is the end of it. It's a really neat thing to learn. I won't spoil it for you, but highly recommend the tour where it will probably be explained.
ETA: I hope I'm talking of the same picture. I know it's in that mirror/window at the end just before you turn. The picture is in the very bottom left hand corner. It's not the groundskeeper though, so maybe I'm thinking of a different picture? Hmmm.