Thomas Noble’s Images, Iconoclasm, and the Carolingians looks quite good. A significant portion of it is available on googlebooks, and I’ve looked over as much of it as is available. His treatment of Frankfurt is very helpful. He notes that Frankfurt was:
1) A long time coming. Alcuin and then Theodulf composed lengthy theological writings on the issue of images. It was seen as authoritative for the churches within the Frankish empire.
2) Not wholly dependent on the faulty translation of Nicaea II. Though they did use the portion on “adoration,” this was not all there was to their case. It merely seemed the most outrageous statement among many bad statements, and the translation they were working with was the official one for the West.
3) Inconclusive for the West’s future. The Pope was pro-icons, but, in the words of Noble, he “agreed to disagree” with the Franks. He knew that they had very different views, and he knew better than to directly rebuke them. The Franks, likewise, did not wish to wage war against the papacy at this time. This compromise did, however, effectively state that Nicaea II was neither “universal” nor “ecumenical” in the Western mind.