For one thing, Iconoclasm would have thrived in Islamic dominated territory… but it didn’t. The first out break of Iconoclasm began in Moslem territory, though this was not Christians destroying images, but Moslems destroying Christian images (Pelikan, p. 105). There is also reason to think that Moslem influence inspired the Iconoclastic Emperors (for one, all of them were from parts of the Empire in which Moslems had made inroads), but the fact of the matter is that the only part of the Church in which Iconoclasm took hold was in those areas in which the Iconoclast Emperors could impose their heresy upon the people. In all areas of the Church beyond the reach of Byzantine arms, the Church opposed the Iconoclasts and broke communion with them. One of the most vocal opponents of the Iconoclasts was St. John of Damascus, who lived under Moslem rule, and suffered persecution as a result. If the Iconoclast view were really the traditional view, we should have expected to see this opinion dominate the Christians living under Moslem rule. At the very least, we would expect some Iconoclasts to speak out from among these Christians, but in fact, the opposite was true—there were no Iconoclastic voices heard from Moslem dominated lands, despite the obvious advantages such Christians would have had with their Moslem rulers.
Also, prior to the Iconoclastic controversy, we have extensive archeological evidence that Icons were used throughout the Church, and were this a departure from Apostolic Tradition we should expect to find a huge controversy on the subject from the very moment that Icons first came into use, which would have only intensified as their use became more common. We find, however, nothing of the sort. In fact, thirty years prior to the Iconoclastic controversy, the Quinisext council established a canon regarding what should be depicted in certain Icons, but hasn’t the faintest hint of any controversy about Icons per se:
“In some of the paintings of the venerable Icons, a lamb is inscribed as being shown or pointed at by the Precursor’s finger, which was taken to be a type of grace, suggesting beforehand through the law the true lamb to us Christ our God. Therefore, eagerly embracing the old types and shadows as symbols of the truth and preindications handed down to the Church, we prefer the grace, and accept it as the truth in fulfillment of the law. Since, therefore, that which is perfect even though it be but painted is imprinted in the faces of all, the Lamb who taketh away the sin of the world Christ our God, with respect to His human character, we decree that henceforth he shall be inscribed even in the Icons instead of the ancient lamb: through Him being enabled to comprehend the reason for the humiliation of the God Logos, and in memory of His life in the flesh and of His passion and of His soterial death being led by the hand, as it were, and of the redemption of the world which thence accrues” (Canon LXXXII of the Quinisext Council).
Aside from this, there are many other things about the Iconoclast which show the novelty of their heresy: they opposed monasticism, despite the fact that it had unquestionably been embraced by the Church for centuries, they were found of robbing monks, taking their land, and forcing them to marry, eat meat, and attend public spectacles (and those who resisted often were the public spectacles), contrary to well established monastic practice. Even Protestant historians are forced to concede that the holy men and women of the day were supporters of the veneration of Icons, and that the Iconoclasts were a rather immoral and ruthless lot.
“Much has been written, and truly written, of the superiority of the iconoclastic rulers; but when all has been said that can be, the fact still remains, that they were most of them but sorry Christians, and the justice of the Protestant Archbishop of Dublin’s summing up of the matter will not be disputed by any impartial student. He says, “No one will deny that with rarest exceptions, all the religious earnestness, all which constituted the quickening power of a church, was ranged upon the other [i.e. the orthodox] side. Had the Iconoclasts triumphed, when their work showed itself at last in its true colours, it would have proved to be the triumph, not of faith in an invisible God, but of frivolous unbelief in an incarnate Saviour.” (Trench. Mediaeval History, Chap. vii.) The Seven Ecumenical Councils of the Undivided Church, trans H. R. Percival, in NPNF2, ed. P. Schaff and H. Wace, (repr. Grand Rapids MI: Wm. B. Eerdmans, 1955), XIV, p. 575, cf. 547f.
One can only be an Iconoclast if they believe that the Church can cease to exist—contrary to the Scriptures—because there is no doubt that the Church rejected Iconoclasm and used Icons from at least as far back as its use of catacombs (which are full of Christian Icons). This is an option that thoughtful Evangelicals generally reject (see, for example, A Biblical Guide to Orthodoxy and Heresy, Part Two: Guidlines for Doctrinal Discernment, in the Christian Research Journal, Fall 1990, p. 14, section 3, “The Orthodox Principle”).
For a thorough history of the reception of the Seventh Œcumenical Synod, particularly the mistranslations of many of its decrees in the West, and the resulting misunderstanding of the council on the part of the Franks, see The Seven Ecumenical Councils of the Undivided Church, trans H. R. Percival, in NPNF2, ed. P. Schaff and H. Wace, (repr. Grand Rapids MI: Wm. B. Eerdmans, 1955), XIV, pp. 575-587.