This canvas art shows the Night King is riding a horse, and the whole area seems that everything is covered with snow. It might be, because the winter is already arrived.
White Walkers will get more stronger in the winter season. Behind the Night King there comes the Viserion Dragon (The Ice Dragon). Ice Dragon, Dany’s dragon Viserion was raised from the dead by the Night King and the it became the main key for Night King, So he can command and control a dragon and destroy larger amount of living.