What would I do? I would add a decorative mirror above each. I feel like different art would compete too much with your horses. Mirrors will not.
These mirrors (from Uttermost) range from between 18 and 21 inches square. Anything in there would be a good size.
You could also do four mirrors- with two on either side- like the way the art is hung below. You just want to make sure that you are well below the top of the horse picture- so I would measure it and draw it out before purchasing.