The images adorning the walls of the church are called icons. The most prominent ones you will see are of Jesus Christ, and Mary, the mother of God. Other icons represent faithful Christians that are now departed.
Orthodox Christians utilize icons in worship as a way of calling to mind examples of people who demonstrated their faith in God throughout history. Icons have been a part of traditional Christian worship from the very earliest days of the faith.
During a service you will see people bow before certain icons, and even kiss them. This is not idolatry or the worship of images. It is a way for Orthodox Christians to venerate, or demonstrate our love for, the people the images represent. In the way someone might talk to or even kiss a photograph of a loved one that is far away from them, we pay honor to these examples of Christian faith.