Personally? I would hire art installation services and fill the stairwell with artwork. It would be stunning.

But that's me... I can't stand bare walls. And to me, art needs to be personal, it needs to have meaning, not be something picked just to fill empty space... so if you have a lot of framed artwork that you love... think about using it in that space.

The wall needs to be smooth as glass for wallpaper to look right. No texture.

Whatever you do, just make sure there's no glass on anything at the bottom of the stairs going to the landing. If someone trips, you don't want them smashing into glass.

paint should be nice..

I would paint all three walls a darker color like previous people have suggested, I would also maybe add more art to the walls then just that one piece.

I believe a light gray or light blue paint would be good, and you could look up some sponging techniques that could make it pop

https://goo.gl/images/HNvUKV sort of like this.

paint it. any dark colored paint would do as long as it looks good for your eyes.

I agree with Verona to paint all three walls with a darker color and put some paintings as well so that it'll be balance.

Yes, I suggest to paint the wall rather than putting a wallpaper.

And if the stairwell is too dark, it may be hard to maneuver, unless you keep lights on all the time.