You MUST consider not only the formal elements (e.g. Tone, shape...) and their detail, but the piece's meaning- its background. Look at the time period that it was created in, or the art movement that it was created during. For example, in 'An allegory of the vanities of human life', the shell represents wealth, as a shell was a very expensive item at that time, whereas now it would represent only something as simple as the sea. Hope this helps..
