I would try to ignore the fact that the wall is on a slant. If you have a chalk line, level a line at the lowest point of the ceiling and straight across. You can use that as your guide. Here's a few ideas for you:
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Also, do you have kids? A window that close to the floor would not even be legal in California. I would want something in front of it, like this: [houzz=]

Thank you Julia! The orchid photos are too small I think - but I really like your idea about a large or 2 large black/white prints. And yes, I've always hated the frame for the other piece of art - my mother in law gave it to me and it's a Tarkay limited edition print so I love the art itself - I think painting the frame is a great idea - or if my husband thinks it would hurt her feelings i should probably move it. I also hate the curtain but am stuck with it for now as it provides privacy that we need in the evenings. :) It will be replaced one day. :) Any thoughts about adding some color to the space? Perhaps through an area rug near the console? Thanks so much for your input!!

I would lower the wattage in the two lamps - much too much light. Good advice from others on the pictures. A matchstick blind or a Roman shade would work on your window and give you privacy. Even a shere on a tension rod would look good.

I'm a big fan of neutrals, and black and white, which is probably why I love your space so much. I'm the wrong person to ask about bringing in color. ;)

If you feel a need for color, what about a large floor plant in the corner where the curtain is in a black ceramic pot. It must be a real plant, and you'd have great light there for it!

Maybe you could bring in some color in a floor mat at the foot of the stairs. I'd make it graphic, something to echo the grid of the rail, no more than two colors, with black being one of them.

When you get new curtains, I'd get them in a tone close to the wall color, textured, tone on tone, or a very small print. If you sew, you could band them with wide black grosgrain ribbon.

Keep it simple. No clusters of small pictures. Let your rail be the focal point of the hallway.

Thanks to everyone for your input!

oldbob - yes, I have a son, but assure you the window is extremely thick and very safe [ for example, my 140 pound dog has jumped up onto in over and over to bark at the pizza guy and no problems ;) ] but the built-in bench is very nice looking but don't think there's enough space on our landing.

julia - thanks again for all the good suggestions!

houssaon - thanks for window treatment ideas and lighting input. I really appreciate everyone's ideas - love Houzz community!!

Just one large piece as suggested previously. Locate at bottom of stairs. Add coordinating rug at bottom of stairs.

Thanks for you idea Dar - I'm not sure what you mean by "locate at bottom of stairs" - do you mean you would do a tall/long piece and put it closest to the window or hang a large piece low? I'm not quite sure how to picture your suggestion and would love you to elaborate if you don't mind. Thanks!

I think this would look nice.

The pictures I sent are big but all you have to do is make just use half of the picture placement idea.

Or there's this great Martha Stewart way: http://www.marthastewart.com/272930/picture-an-easier-way

Thanks again for everyone's input! Now . . . to find some art!

Wallpaper behind it would add to your stairwell