What is (visual) art?
It is any 2D image or 3D object that is made by the artist for the appreciation of other people. By appreciation I mean whether or not the viewer "likes" what they see. What is meant by likes I will leave to the neuroscientists to explain.
Art is underpinned by two qualities: craftsmanship and originality. Both of these can be assessed objectively; however a knowledge and experience of creating art is required to properly assess the former and a knowledge of art history is required to properly assess the latter.
These two qualities, however, will not necessarily influence whether or not the viewer likes the artwork. This liking or not liking is subjective and cannot, by definition, lead to an objective claim as to the worth of the work. Only by taking account of the views of all viewers can any claim to the artwork's worth be made. If a high percentage of the viewers like it then this is the closest you can get to saying it is a great work of art, and, similarly, if a high percentage don't like it then it is a bad work of art. This is because art is created for other human beings to view and appreciate.
It is entirely possible, then, for an original and well-crafted artwork to be bad, as well as for a shoddily done, unoriginal artwork to be good. This subjective appreciation of art appears to be primitive or visceral and is very difficult to rationalise.
If an artist consistently produces art that is well liked then they are a great artist, whether or not they are a great craftsperson or posses originality. (Goya's later paintings, done after he had been sent silly by the lead in his paints, are better liked than his earlier ones despite being less well painted.)
A photographer (nowadays) will always fail the two objective qualities of art creation, because taking a photo is (usually) easy and because photography is much more limited in what it can produce, so it is almost impossible, now that so many photos have been taken, to be original. But if a photographer can consistently produce photos that a high proportion of viewers like then they can be considered great artists within those limits.
No artist is so good that every viewer will like their work and no artist so bad that nobody likes them, so there will always be pointless arguments over whether any given artist is any good.
The above is written in didactic style for (hopefully) clarity and brevity, though it is, of course, open to discussion and argument.

I was prompted to write the above by so many prudes leaving nasty comments on deviations that depict nudity. They claim that it is not art. This is not true. A visual artwork is any image or object made by an artist for the purpose of viewing by other people. (This means that any non-functional decoration on a building is also to be considered art.) To say an artwork depicting nudity is pornography and not art is as stupidly pointless as saying that a landscape is scenery and not art. These prudes are, of course, entitled to offer their subjective opinion of the work, and if they posses the above mentioned requirements they can opine on whether it is well made or original, but these people seem to think that their opinion is the be-all and end-all of art appreciation. If most people agree with them and don't like the artwork then it will be a bad work of art (and one hopes that the depiction of anything truly disgusting will be disliked by most people).
But an artwork can depict anything. And since it is made to be seen by humans, then if it depicts sexual activity it will be of a greater or lesser interest to any adult viewer, since sexual activity is of interest to all humans to a greater or lesser extent. In prudes who leave these objectionable and offensive comments it would appear to interest them to an unhealthily great extent. (As an aside, I recently came across a female DeviantArter whose gallery of favourites consisted entirely of Star Trek images with the exception of one or two per page which showed a man in the nude, or sometimes just his dangly bits. Star Trek obviously did it for her, but since there is no sex in Star Trek she had to assuage that perfectly normal and healthy interest in (in her case) the opposite sex by adding pictures of nude men.)
If, however, you like an artwork that depicts an act of violence (including rape), then this might suggest you have a predilection for violence and are to be viewed with suspicion. Yet art that depicts violence is rarely complained about (unless it is sexual violence).
But the viewing of (non photographic) art, no matter what it depicts, does not do anyone any harm. For a book-length treatment of this subject see Alan Moore's "25000 Years of Erotic Freedom". Prudes are trying to suppress something harmless in other people. They are interfering, neurotic, killjoy busybodies and they should fuck off and live with the Taliban.