The lovers frozen in their position on the urn is symbolic of the difference between art and life. They will never consummate their lives and live on as mortals will. While there is beauty for the lovers, the only truth is this one moment of theirs, while mortals can be fertile and not sterile in their love as they are.
Also, the different pictures on the urn represent time. By this, I mean that the picture of the lovers, for example, is a moment frozen in time. This beautiful moment is one that is captured for all time. It is timeless and cannot be changed.
The urn itself is a symbol of art and beauty.
The lovers in an "almost kiss" are symbolic of love.
The stories on the urn...the lovers, the tree, the musician, the priest and the heifer, the empty town...are symbolic of a truth we, the future, may not completely learn.