I think that there should be a bit more background detail, it would bring more depth into the picture, a bit of foliage would do it perhaps?. Also, I think the turtleneck so to speak, should be a bit less spiky, leafy, (whatever) from this angle it kinda looks odd.
I absolutely love the blueish haze behind them, gives a bit of much needed contrast. Thranduil's hair and the elk's fur look simply fantastic. Deviantart is flooded with standalone pictures of Thranduil and ones of him atop the elk. This is a welcome change of pace. Overall, a fine piece.