Wow, that is definitely an awkward space to decide on art for! I would say because of that slant where the ceiling meets the wall, it is going to be hard trying to make a square piece work. I would consider two long, skinny rectangular side-by-side pieces, or as Travis Interiors said, a round piece or a nice blue color. Maybe a blue mirror, or cluster of small mirrors. But I think going blue will be a nice twist to the neutral space, something that yellow does not provide. Whatever you do, I think it's a perfect space for a cluster of really cool accessories for the sides!!