I've always read/heard that accent walls (whether painted or papered) should end on the inside corners. I'm not sure this wall is a good wall as an accent. :-(

Hopefully others will chime in soon. <3

(I partly wanted to post to bump up your thread, and also because I'm curious on the accuracy of the design information I have, lol.)

I would not take away from the piano with wallpaper. IMO picking up the dark color of the dining chairs and applying it to the wall as a background for this sheet music graphic might be a better composition.

Comes in multiple sizes.
https://www.etsy.com/market/sheet_music_wall_art

No. Change the art, the bench, or change the color of the room entirely. It's got no reason for an accent. It's a piano. : )

Depending on the look + mood you are trying to create, there are so many ways you can go.

All of these are not necessarily a match for your space - but just a few ideas/views:

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More ideas here: https://decortango.com/piano-room-decor-ideas/

I like the pieces you have but a bit larger , black frames and be done. As for a wall paper I would need to see the rooms next to this to even have a clue what wall paper would work.