I like two larger frames, versus more smaller ones. I'm not a fan of prints from famous painters, but agree that a floral would be pretty. Have you considered putting fabric in the frames? You can find any colors you want and it's inexpensive.
What about a big dramatic mirror? If it had a similarly stained frame and was hung on wires tilted down a bit, could it overlap and possibly rest against the molding in the back? I like the smaller items on the other walls because they are smaller walls, but something big enough to take on that large wall just seems out of place hung above the rail. That's really hard.
At our local paper/craft store they have sheets of wrapping paper with a whole US and World Map on them...only $5 per sheet and I bet they are even larger than the ones that you have photoshopped into the frames.
You might want to look at the designs/styles of single/hand-made wrapping paper sheets in your town and you might find something that would work. (I think they had some other adult looking designs and pictures also.
Is the rail all through the house? Could you just lower it? A prybar, some sanding, and some paint would be a weekend project.
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It's not throughout the house, but we found out when we were redoing the electrical that it's there to disguise a seam between plaster and drywall. We were guessing there was some kind of damage or renovation and they were lazy about the repair. Moving it would be a huge project.