I have just concretely figured out why Garuda is always drawn in a superior position. I have an account on ARTSTOR (an archive for pictures of art) through my school, and I was just browsing through the pictures of statues, textiles, and paintings of Garuda. Each picture comes with accompanying information about the time, place, creator, media, and (if applicable) the position. Well, I noticed that the oldest statues of Garuda in said superior position, many of which predate the aforementioned Tibetan Buddhist motif, were located on load bearing / decorative
lintels and archways. These structures decorated the entrances to the Hindu and Buddhist temples of India and other countries. Here are a couple of examples.
This is the oldest example that I can find (thus far) of him above an arch. This is from the archway of an Indian Buddhist Chaitya Hall in Maharashtra dating from the 2nd-3rd century BCE. He is the one on the left.
This is from an interior lintel of a cave temple dedicated to the Hindu god Vishnu in Karnataka dating to 578 CE.
This is from the lintel of a temple dedicated to the Hindu goddess Durga in Karnataka dating to 700 CE. Notice the Roman influence.
This from the lintel of a (Buddhist?) temple in Cambodia dating between 875-925.
These are just a few of what I have found. Please keep in mind this photo that I posted earlier of what I thought was Garuda in Yungang cave #12. As you can see, the bird is located on a decorative-type lintel above the three bodhisattvas’ heads, so this is most likely Garuda.
Therefore, it appears that the Tibetan Buddhist motif of Garuda being in a superior position above the head of the Buddha or other Buddhist figures in paintings and textiles was spawned from ancient Indian Buddhist and Hindu architecture. Any deity placed above a doorway, archway, or lintel acts as a protective guardian.
Edited by ghostexorcist, 13 October 2011 - 07:31 PM.