By the 4th century we have evidence of the veneration of material objects, holy relics etc. associated with martyrs. You see this for example in the writings of Gregory of Nyssa.

At the same time you have the development of the imperial cult, translated and incorporated into the Easter Empire's practices. One element of this was that the symbols, and the image, of the emperor substituted in some sense for his presence, and were to be accorded the same kind of respect as the emperor would. This kind of thinking seems to have carried over to the religious practice, and by Augustine in the early 5th century, you have awareness that some people are using religious images in worship.

The practice seems to have grown significantly in the 6th and 7th centuries. For example, in a debate in 656 we read that the two speakers kissed the Gospels, the Cross, as well as icons of Christ and Mary.

Right through the same centuries you see theological uneasiness and opposition to the use of icons among some writers, but in the 8th century this blows out into a full-scope theological argument (the Iconoclast controversies).

It's in this period that you get a refinement of the 'theology' of icon veneration. One of the eventual outcomes is the victory of iconodule (the supports of using icons) theology on the matter, especially in terms of delineating a difference between icon, and the person imaged, and veneration to the object, and (true) worship offered to the person imaged. From the mid 9th century onwards icons were securely established in the Orthodox church and has been a pretty much unchallenged staple element since then.