A couple of things to keep in mind:
1. Do not have your one item hit the background item at it's mid-point. You do not want to cut it in half. Try to have it land at the 1/3 or 2/3 point. (most likeley the 1/3 point when you have only one item)
2. You do not want your one accessory item to be too large- this will make it feel off balance.
3. I would have done this fireplace just a little differently- I would have larger greens, that overlap the picture a bit. Not higher- it hits the picture at a good height, just fuller, or longer- so that it overlaps. I think this would tie the items together more, and they would appear less disjointed and separate.
Thanks to Decor fall winter 2007 for the photo!