A "stage picture" is a term usually used to refer to the particular tableau formed at any given moment by the particular placement (called "blocking") of actors on the stage. It's a term usually used by directors, who are among other things, responsible for the "stage picture."
A stage picture is what the director tries to create for the audience by careful placment of people on stage. Sometimes the lead is down center, or he could be to one side balanced by other actors on the other side of the stage. This whole "stage picture" idea is usually to get a point across using people in spacial relationships on the stage -- lovers kissing says one thing, but lovers on opposite sides of the stage with their backs to each other is a very different stage picture and sending a very different message.