After looking at some more pics of original Space Invaders cabinets, my theory is now that the invaders primary means of locomotion is to walk on air. During the course of the game, they walk down through the sky at about a 45 degree angle from the ground. Exactly how they do this, is a mystery.

Links:
Coin-Op Museum
coinop.org

My earlier comments about the extent to which all later games were influenced by the actual game graphics, and not what they were supposed to represent, still stands.