My grandparents, who were avid birders, preferred those bird books with color illustrations to those with color photographs. The reason was because the illustrated great kiskadee (or fox sparrow or scrub jay, etc.) looked more like any great kiskadee they would see than did the photograph, which was of a particular great kiskadee. The illustrator has done the work of creating an image -- an ideal, or concept -- of the great kiskadee, which is impossible to do with a single example. To create a concept, you must have more than one example of the object being conceptualized, and a conceptualized object makes other similar objects easier to identify. But if you have a single example to work with, you are uncertain if a similar object really is of the same kind as the object of comparison.
We create concepts -- ideas, or Forms -- by taking a set of similar objects and mentally subtracting the differences. We then give that set of objects a label to help us hold them all together. Metaphors are necessary to reorganize our conceptual categories. They create new overlaps, stretch our conceptual categories, and push us into new territory. All art is metaphorical.
All art too is conceptual -- in part. More accurately, art lies on the borderland between the conceptual and the perceptual. Even the most accurate photorealistic painter doing a portrait necessarily has in mind the faces and bodies of every person (s)he has seen. The painting contains within it the tension of representing the object painted and representing the concept of the object painted.
A concept is a concept of something. Those abstract artists who claim to be painting "pure concepts" are thus doing nothing of the sort. One can paint a conceptual tree, a conceptual dog, a conceptual horse, chair, or person -- or even geometrical shapes representing higher-order concepts/abstractions -- but what would it even mean to paint the concept of concept? (A paradox arises here: doesn't the concept of concept necessarily contain itself?) A painting or other work of art is always representational -- concepts are mental. Thus, any work of art is the best that artist can do to represent the meeting of the mental/conceptual and the real/perceived world.
We use representations to communicate to each other. A word is a representation of a concept, and a concept is a mental representation of a set of various similar objects. My set is not your set, so what we communicate is never without some noise or error. If I say "tree," I may be thinking of maples and you may be thinking of pines, so there will be some level of miscommunication. We also have different emotional tags attached to concepts (which include experiences). I will experience Turner's "The Shipwreck" in a different way than someone who has been in a small boat on stormy seas. BOth experiences are equally legitimate, though each will result in different interpretations.
Art is thus a kind of language, as it communicates information from one person to another. Art too is most beautiful when it achieves the balance between the conceptual -- which is one -- and the perceptual -- which is many. This is also reversed, and this reversal is most evident in art -- because the concept is one made of many, and art represents one thing being perceived by the artist. A beautiful work of art represents this tension between the unique object and the conceptual one derived from the subtraction of the uniquing elements in each individual object in a set. This tension is necessarily present in what we typically think of as being representational work, but can be lost in the most abstract works. It is also lost in those works that push toward the perceptual/reality. Such works may not lack in beauty from other paradoxical tensions inherent in the works, but such works won't be as beautiful as they otherwise could be. They may not even be works of art.