What is iconography and how did the Church use it to support its ideas and teachings?

Icons are religious works of art. They are primarily used to illustrate holy figures such as Jesus Christ, the Virgin Mary, and saints, as well as morally uplifting scenes from the Bible and saints' lives. The word icon comes from the Greek eikon meaning "image." They form a major part of worship in Christian denominations such as Roman Catholicism and Eastern Orthodoxy.
Icons originated at a time when the vast majority of people were illiterate. Even the relatively small number who could actually read were denied the opportunity to peruse sacred scripture, as it was thought by the Church that access to the Bible should be restricted to those in holy orders such as bishops, monks, and priests. So icons fulfilled a useful function in transmitting the Christian message to people who would otherwise have no way of receiving it.
Icons are not just works of art, however; they are sacred artifacts, objects of veneration by the faithful. The deep honor and respect assigned to each icon pass over to the original archetype which they illustrate, be it Jesus Christ, the Holy Mother, or the saints. To be sure, Christians who venerate icons are not worshipping them. Icons are not intended to be a substitute for God; they're a means of worshipping God, a visible image of something inherently invisible. The faithful believe that icons bring us closer to God and his saints, and therefore form a bridge between the temporal and the eternal, the imminent world and the transcendent.