Mary, it depends on what appeals to you. This link is probably more info than you need, but I like the art in the first photo. You can use the firebox opening as a size guide for the art. You can look here on Houzz for art, and Ballard Designs is another source you might check.
https://www.houzz.com/magazine/dos-and-donts-of-hanging-art-above-a-fireplace-stsetivw-vs~71949227
I've kept this photo on file since I like the way this mantel is done. I know it's completely different than yours, but the simplicity is there.
Q
Pink and blue
Q