This picture shows one way to go. Get a large mirror and be done with it. Easy. Classic. This mirror goes with the room but to my eye, I probably would have made the mirror a little narrower. It works because they took the mirror right up to the ceiling. If they had used a painting this same size it wouldn't have worked (probably.) The mirror is nice because it reflects and doesn't become just an extension of the "box" below it.
This is an interesting fireplace. That exaggerated tile face determines a lot. AND the television placement dictates what you put on top of that mantel.
If you have tall ceilings and a big fireplace/mantel then you have to think LARGE with your decor. That big piece of art may seem too dominant but if done correctly that one piece can really make your decorating job easier. If you want multiple objects then you have to find big, over sized vases, statues, etc... A good decorator can help! I know it is hard to navigate the sea of endless accessories!
You can add impact with small objects just by placing them en mass. If, for example, you were to put just 3 little candles out it would emphasize how small your mantel is. Going overboard can help it look fantastically deliberate! Hope I've helped. It was difficult to find good photos for you, so email me if you want help on an individual basis. Happy decorating! I love this time of year for mantel decor!