Answer : It is quite difficult to answer the question without looking at which specific art has to be considered. But few generalizations can be made from Byzantine art from his mosaics.
1) They are illustrative. Byzantine artists would paint to show a person or an event. Typically, a saint, or a bible story, or perhaps an important event associated with an Emperor.
2) The pictures are usually stylized, with standard form and imagery. If considering a picture of a saint, there will be certain standard images and symbols associated with that saint so anyone looking at it. Their status will be shown by haloes, there will be objects or signs next to them, and the picture will have a use. Either it is to aid worship, or teach a story or - another type of Byzantine art - may even show a map.
