As per wikipedia article on Vaikhanasas:
Vaikhanasas claim to be a surviving school of Vedic ritual, the Taittiriya Shakha of the Krishna Yajurveda. Vaikhanasa tradition says the sage Vikhanasa, who was a manifestation of Lord Mahavishnu, had the Upanayanam along with Brahma consecrated by His father Lord Mahavishnu and educated under Lord Mahavishnu all the Vedas and Bhagawat Saastra on how to worship SriBhagawaan in His Archaavathaara - Iconic Form.
My question is which form is it? Is there any detailed description of this form mentioned in scripture?