One needn't know who first coined a word or even how it was originally used for that word to be meaningful, and similarly the fact that the origins of ancient artworks are murky doesn't entail that they are without meaning. The original meaning may be lost, but new meanings are generated, often retaining traces (often more) of earlier meanings. Now, of course, some words are more commonly understood than others, and there are lots of artworks that hold generally shared meanings for people. Sublime landscapes, beautiful portraits, and rousing political artworks support common interpretations galore. So, it seems pretty clear to me that meaning is transmitted and shared through artwork. Sure, when pushed different people generate different shades of meaning and different connotations when asked about how they understand words, but the agreement, facility, and approval with which people share word usage points to shared meanings. And some words are understood only within recondite discourses by small audiences scholars and technicians. So it is with a some artwork, especially the most avant garde and experimental. Poets often twist and strain the meaning of words, which can make shared meaning difficult, but often not impossible to tease out. The meaning of paintings is the product of a conversation between the painter, the audience, and critics, as well as other painters. That meaning can change over time, or not. I might add that I think, just as it is with words, it's not exactly right to speak of a single meaning for an artwork. One remarkable property of good art, like powerful language, is how fecund it is, how much meaning and different meanings it generates.
A painter painted a masterpiece to express his observation of the world. However, everyone's understanding of the same works is quite different, and the painter is not sure whether his or her works are reflected in others. The original meaning of some ancient paintings has long been unknown, or gradually distorted in the history of interpretation. What is the meaning of painting? Just speak to oneself? Can the meaning, or beauty, be transmitted among different people?