The information contained in artworks created during a specific time period in history are, of course, the product of an artist of that specific time period. The Archaic-Classical Greek period, was a time when change was welcome and when dicta in philosophy, the arts, and even fashion were established. This being said, the first-hand account of the Classical Greek artist would document these changes, especially the comparison and contrasts of one style and another.
Therefore, the information contained in the artworks produced by artists of a specific time period are considered first hand accounts of life in that time. These are considered primary sources.
Basically, primary sources consist on any item which comes directly from someone who lived in the time period and produced any form of testimony or documentation associated to the events that occurred then. This solidifies its validity and relevance to the study of Ancient Greece.