buy multiple pieces from the same artist. The two beach landscape photographs in this Architectural Digest España room are by Elger Esser. Because they're the same size and in the same frame, they work together perfectly.
Horizontal, black and white photos are custom framed in vertical mouldings with super long matboards. Just goes to show - don't let the orientation of the art determine where it will and won't fit. Anything's possible with custom framing.