A really beautiful article from The Hindu about the various postures of Lord in different temples.
If we visit a house, and the lady of the house says that her husband is away, and will be back only the next day, we wait for his return. And since we have to wait a long time, we lie down on the pyol at the entrance to the house. If the man will join us shortly, we sit outside the house, waiting for him to join us. If the man is ready and will be with us in a few minutes, we await him standing.
Just as our posture changes according to the situation, Lord Narayana’s archa from also is seen in different postures in different temples, depending on the circumstances. Lord Srinivasa of Tirumala is seen in a standing posture, because He is not content with the fact that there are reformed souls, whose only thoughts are of Him. He wants to bring under His fold every one of us. Imagine a greedy man who has a sack full of coins. Even if one coin drops out of the sack, he will not go his way, thinking that after all when he has a sack full of coins, a single coin does not matter much. He will stoop to pick up the coin. So also is Lord Srinivasa greedy when it comes to saving us. There may be many who adore Him, but He will not let go those who have not realized the path to salvation.
It is in order to instantly lend a hand to the person who slips from the path of righteousness, and put him back on track, that Lord Srinivasa is in a standing posture in Tirumala.
The complete article can be read here – www.hindu.com/2009/10/19/stories/2009101960191200.htm