The iconoclast council of Hieria in 754 defined its doctrine at length, and the victorious party destroyed what it could of the record. The definition survives entire — because Nicaea II read it into its own acts in 787, clause by clause, in order to answer each one. The most complete iconoclast text we have is preserved inside the document that condemns it, which is a common enough way for a losing argument to reach us.
The Seven Ecumenical Councils (NPNF series 2 vol. 14) · NPNF2 XIV — Nicaea II session 6