One immediate and superficial observation about what we may learn from the controversy, at least with respect to those today who fling charges of idolatry at Catholics and Orthodox over images, is to say, "been there, done that, got the conciliar endorsement." In short, I've never heard an argument against images today that wasn't already advanced and addressed over a millennium ago. It's nothing new. Unfortunately there must be more to our answer to the modern despisers of images than simply appealing to an ecumenical council, since (on average) the modern iconoclast sympathizer will not be interested in what councils have to say when they differ from his own theological tradition's views.
(As a fascinating aside, the old Catholic Encyclopedia reports that the Iconoclast controversy began with Nestorians managing to persuade Leo the Isaurian to start persecuting iconophiles. Once again we see Nestorians afoot; once again we are reminded that some Protestants have been charged with Nestorianism.)
One thing Pelikan reports strikes me as interesting in view of other facts.
Constantine V [asserted] that a genuine image was 'identical in essence with that which it portrays.' The term used here, 'identical in essence [homoousious],' came from the trinitarian language of orthodox dogma...This definition of the relation between the image and the thing or person imaged, which 'undoubtedly was characteristic not only of Constantine but of all the leading minds of iconoclasm,' meant that an image of Christ being used in worship was in fact the 'falsely so-called image of Christ,' since it obviously could not be 'identical in essence' with the person of Jesus Christ himself; not even the most vigorous defenders of the images maintained that it was. The very definition of a true image necessarily implied for the iconoclasts that no painting or statue could ever be an image of Christ. [p. 109]This reminds me of something that we moderns find hard to imagine but which, apparently, was literally true. I'm having trouble locating the reference, but I think that it was in the works of Justin Martyr. As I recall, he pointed out that the idolaters of old really did consider their idols - works of wood and stone - to be literal gods. It wasn't just a question of them thinking that the idol represented the god; it was that the idol was itself the god. I was pretty shocked by this, but really it makes complete sense with Isaiah 44:13-19!
The wood carver takes his measurements, outlines the image with chalk, carves it with chisels, following the outline with dividers. He shapes it to human proportions, and gives it a human face, for it to live in a temple. He cut down a cedar, or else a cypress or an oak which he selected from the trees in the forest, or maybe he planted a cedar and the rain made it grow. For the common man it is so much fuel; he uses it to warm himself, he also burns it to bake his bread. But this fellow makes a god of it and worships it; he makes an idol of it and bows down before it. Half of it he burns in the fire, on the live embers he roasts meat, eats it and is replete. He warms himself too. "Ah!" says he "I am warm; I have a fire here!" With the rest he makes his god, his idol; he bows down before it and worships it and prays to it. "Save me," he says "because you are my god."This passage makes no sense unless that wood carver literally believed that his block of wood was his god: not just a representation of it, but really his god. Why else does Isaiah say that such a man has shut his heart to all reason? Does such a thing make any sense? Of course not. But this is apparently what they literally did. And this is consistent, it seems, with what the iconoclasts' idea of an image was, as seen in the quotation from Pelikan above. Now the only problem with this, when it comes to the Catholic view of images, is that no Catholic says any such thing about an image of Christ or the saints.
They know nothing, understand nothing. Their eyes are shut to all seeing, their heart to all reason. They never think, they lack the knowledge and wit to say, "I burned half of it on the fire, I backed bread on the live embers, I roasted meat and ate it, and am I to make some abomination of what remains? Am I to bow down before a block of wood?" [JB]
This is why, for one thing, it cannot be said that Catholics violate the first commandment. When we do not worship images; we do not serve them. What we pray before a statue or icon is directed not to the stone or paint, but to the Lord represented there, or to the saint represented there. The first commandment is not concerned with such things, but rather with the literal worship of the literal image.
The modern iconoclast has a problem, it seems to me. He would have us believe that any veneration before an image - whether by an idolater literally worshipping the stone, or by a Catholic offering veneration to the one represented in the image - is idolatry. Well, how then could such a man offer veneration to a king or emperor? It is beyond all argument that men in all ages have rightly and necessarily and justifiably knelt in veneration of their rulers and other betters. If it is a sin to venerate him who is represented in an icon, would it be any less a sin (on this reckoning) to venerate a mere man?
There is no sin intrinsic to the use of images and statues. A mere appeal to Exodus 20 is insufficient to say otherwise. Historical context demonstrates clearly what is forbidden in the first commandment, and it is not praying to God or the saints while kneeling before an icon.