The art work is an expression of human being. And an artist follows his or her passion, feelings when he or she creates the artwork. So the real meaning of the art must be with him. And interestingly sometime artist himself doesn't know. They follow their mood and go with the flow.
But again, its true that the observer can also and (often) does add his own meaning to art. Each mind is unique, and our thoughts and emotions are influenced according to our own evolution of mind. So its inevitable that individual observer will add meaning to art works.
Here is a simile that came to my mind when i was pondering on this issue.
In the truest sense, God is the Ultimate Artist and this Cosmos is His art work. And why He created it, what is the true purpose behind it, is only known to Him, The Originator Artist. But yet again, we who are the observer or experience, we all have different values and meanings to our life (and its allowed by God because that adds more beauty to life and creation as a whole when we find different meanings).
We all create our own meanings of life, work and living. Don't we?
So may be both side of the coin have values in them - the artist have their own meanings, and again the observer or experience also have his own little meaning created by him (and he should be allowed to do so).