THE PILOT AND TWO PASSENGERS OF A WHEEL/SKI EQUIPPED AIRPLANE WERE TAXIING FOR TAKEOFF FOR AN ON-DEMAND AIR TAXI FLIGHT. THE RIGHT SKI CONTACTED A SNOW BERM AND PIVOTED THE AIRPLANE. THE LEFT WING THEN STRUCK A SNOW BANK. THE PILOT INDICATED THAT FLAT LIGHTING CONDITIONS AND LOW CONTRAST MADE DISTINCTION BETWEEN THE SNOW BERM AND THE SURROUNDING TERRAIN DIFFICULT.