LONDON -- An athlete steps up to take his throw -- except he is holding a missile, not a javelin; a pole vaulter soars high, but seems headed for a landing on a moldy mattress; an Olympic mascot's leg attracts some unwanted attention from a passing dog. Banksy, whose works routinely sell for thousands of dollars, and other street artists could hardly let the London 2012 Games go by without having their say -- despite the legal risks.