Pretty house. It looks like you have quite a bit of things on the walls upstairs - you don't have to have something on every wall. At any rate, the shelving is too high for frames. I would take down the piece you have on the wall, and fill the wall with different sized black/white photos...gallery style like this:

Thank you Sammie!

Is that wall seen from the downstairs area?

Only slightly

Oh wait your photos didn't load until after I posted that. Your stair case is already busy with the metal baluster. I would do one large or three large art pieces and call it a day. Anything more will be too busy. In fact I am not sure exactly where the two pieces are in your third photo but even those are too much. I would take them down. The eye needs a place to rest and not every wall needs something on it. Less is more.

Thank you Design Interior South and live_wire_oak!