Fourth Council of Constantinople

Technically, there were two of these synods, both held in Constantinople. One of them was called by Emperor Basil I, with the cooperation of Pope Adrian II, whose support Basil wanted after his recent coup (he had assassinated the previous emperor, Michael III).  So although it was held in Constantinople, it is considered a council of the Roman Catholic Church, not of the Eastern Orthodox Church. The catalyst for calling this council (October 869 - February 870) was to depose Patriarch Photios I, who was appointed inappropriately by Michael III, and to reinstate his predecessor, Ignatios. Clergy who were supporters of Photios were defrocked. Photios himself was incarcerated in a monastery.

There were over two dozen other decisions laid down as canons from this council that carried great weight doctrinally, even thought it was poorly attended; the first meeting had only 12 bishops, and the total in the few months it was held barely exceeded 100 clerics. The council was held in the Hagia Sophia (the illustration is a 16th century depiction by Cesare Nebbia). 

One of its statements was a re-affirmation of the Second Council of Nicaea's support of the use of icons and holy images. It even declared that an image of Jesus was to be venerated equally as the Gospel itself:

We decree that the sacred image of our Lord Jesus Christ, the liberator and Savior of all people, must be venerated with the same honor as is given the book of the holy Gospels. For as through the language of the words contained in this book all can reach salvation, so, due to the action which these images exercise by their colors, all wise and simple alike, can derive profit from them. For what speech conveys in words, pictures announce and bring out in colors.

It went further to declare that holy images of subjects other than Jesus were also considered worthy of veneration:

The image of his all-pure Mother and the images of the holy angels as well as the images of all the saints are equally the object of our homage and veneration.

The Roman Catholic popes were pleased to have the Eastern Orthodox Church looking to it for guidance, and Pope Adrian II got the credit, though he was not a particularly powerful pope, serving exactly five years. I'll tell you more about him tomorrow and his good luck with family connections and his bad luck with temporal authority.