Rome held out on this, for more than just the canon as relates to the depiction of Christ as the Lamb of God. The Synod in Trullo, also had canons that castigated Old Rome for the custom, that had become defacto a canon, that they would not elevate any man to the sub-diaconate, diaconate, or priesthood, who was still living with his wife as husband and wife. The Synod also made ecumenical the Canons of the Synod of Laodiciea, especially those that related to the Consecration of the Holy Mysteries in a Full Liturgy on Weekdays in Lent, this was also objected to by Rome. There was also a Canon of Trullo, which in complete concord with the full 85 Apostolic Canons (as opposed to the truncated 50 Canons that had been translated into Latin from the Greek by Dionysius Exiguus in the 5th/6th century, it's possible he may have not have a complete version or only a partial manuscript of the Apostolic Canons), prohibits the actual fasting, that is, the complete refusal of all food, until after the Ninth Hour, and even then only bread, water, dry food, etc (fasting season foods), that was practiced in Rome on the four Ember Saturdays, instead, only full fasting was allowed on Holy Saturday (as it was still the practice of that time, to celebrate the Holy Saturday Liturgy after the Ninth Hour, and prior to Vespers, thus, in order to take Holy Communion, one would have to fast, as well as the obvious meaning of the sorrow this fasting is meant to invoke in a person for when the God-Man Christ perfectly fulfilled the Sabbath); while abstinence from meat and dairy was the tradition of the church in all the fasting season, Sunday through Saturday, it was not in the concord with the decrees of Trullo, to allow fasting from all food until after the Ninth hour on Saturdays.
Thus, there were a number of issues of praxis that were being addressed by the Synod in Trullo, many of them were Roman. Many of them, as the Canon states (if I recall), state that, that while the previous images were not evil, horrible, monstrous, or necessarily painted with some heretical intention, I assume, the Canon says that the Church prefers ones that are not such. Thus, what may have been acceptable prior, is not acceptable now. It does not mean what was done in the past was neccesarily evil, but, it says:
Having thus welcomed these ancient figures and shadows as symbols of the truth transmitted to the Church, we prefer today grace and truth themselves, as a fulfilment of the Law. Therefore, in order to expose to the sight of all, at least with the help of painting, that which is perfect, we decree that henceforth Christ our God be represented in His human form, and not in the form of the ancient lamb. We understand this to be the elevation of the humility of God the Word, and we are led to remembering His life in the flesh, His passion, His saving death and, thus, deliverance which took place for the world.
Various popes, as I recall, gave varying degrees of assent, with phrases such as "as far as they are in accord to the customs of the Holy Roman Church", or St. Hadrian of Rome seeming complete assent to the decrees. A Spanish Council accepted all the decrees, but, the caused a great degree of controversy of the issues in Spain, and St. Bede the Venerable decries the Synod as one meant to be anti-Roman, etc, etc. However, despite the objections issued by one Orthodox Patriarchate (Rome), as well as some saints in that Patriarchate (St. Bede the Venerable), this itself does not constitute a reason in itself. Beside, even if one argues that Orthodox Church of Rome could 'hold out' on ratifying the decrees (as seemed like was show by the a 'compromise' or 'delaying' action reached decades, or centuries, the date escapes me, at a smaller meeting at Nicea sometime later), the Church of Rome would essentially loose it's vote after the Schism, assuming the 'holding out' argument can be considered a legitimate argument. After that period, any resumption of person wanted to resume a station in the Orthodox Church, using the old Western rite used before the Schism, etc, would have to comply with the current Canonical Tradition of the Orthodox Church, which means, the issues at Trullo, which caused a lot of praxis controversy, and of course, the later uncanonical, illegal, and later heretical filioque addition are removed.
If modern proponents of the Western rite have no problem accepting the Synod of Trullo in its canon that castigate the enforcement of a celibate priesthood, and the Eastern Churches castigation and condemnation of the filioque, and then the later agrandizements of Papal authority, then surely, it would seem, that the issue of Christ depicted in the symbolic form of the Lamb of God, would not be a large question.