Steve's idea is good - but I don't think he explained it fully. Put an extension ladder or step ladder on the landing (or on a step - both feet on the same step and lean it against the closest wall.) (Not the wall you're trying to paint, but the wall at a right-angle to it.) Run a 2x12 (or a scaffolding plank) from the rungs of the ladder over to the second floor (or over to one of the steps on the way to the second floor.) This will put you at the level of the second floor.
You may need a long plank... Hope you're not afraid of heights. If this is hard to understand, post again, I'll draw a picture.