The painting, « Bus Stop » by Earl Moran, is from 1946, so I think this posed photo was taken then, or shortly after. Norma Jeane Dougherty, as Marilyn Monroe was called at this time, posed for Moran in several sessions between 1946 and 1950, usually for photos that Moran would use to paint from.
Original B&W photo.
A version of the painting as published.
Pimpf : Drifting somewhere... 