To create dimension your fireplace mantel decor is to put smaller boxes in front of mirror or center box. These should cover piece a bit in corners and bottom. Use two or three boxes of different sizes, not larger than one third of mirror. Instead of pictures you can use frames with photos of your loved ones or family. In frame story, in this case should be thinner and not very bright.
Now you have pictures in place, you must add movement to fireplace mantel decor.
Best way to do this is with candles of different heights. Fire creates movement and warmth, while height of candles makes eye travel upward, making fireplace look bigger. If you prefer not to use candles, add other pieces of different heights. Another way to create movement is with flowers or plants. Use a few flowers in a tall thin vessel.
Fireplace is a good space for seasonal fireplace mantel decor. Take opportunity to put specific elements of Christmas, Thanksgiving, different seasons or birthday. You can also change some pictures and some detail by others that represent each special day and change as year progresses.