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houssaon It would help to see the rest of the room. You could go to allposters.com for a wide variety of prints and posts that wouldn't be too expensive. I'm sure you could find something there that would be appropriate. Here are two prints of plaques shown in one of the frames available - you can buy different sizes framed and unframed.

Anish Motwani Associates Hello

just put a nice vase with colorful and well decorated artifical flowers in that. it will be cheap and best thing to do and flowers will give you warmth in the house as well

regards
www.anishkmotwani.in

Denise Webb I have several of these throughout my home. Although mine aren't as large I choose candle scones that are a nice filler for such a large space. And some nice candles for decoration only.

pollyannagal This is such a symmetrical arrangement that you should have the same thing in both niche. I agree with Denise's suggestion of candle sconces. Look for something in wrought iron that would link nicely to the curly metal detail in the central niche and would provide depth. Although it would be impractical to light the candles on a day to day basis they could be lit for special occasions and please do make sure you light them and burn them for an hour or so when you first put them in as an unburned candle in a wall sconce looks too artificial.
The niches are very large so go as big as you can with the sconce, or depending on the design use more than one. To group a few or to make a single sconce look more dramatic in the space you may need to mount them on a back board that will serve to frame it and ground it in the space.