The Important Difference Between Art and Sex
Nudity in and of itself isn't obscene, though it is often inappropriate in certain venues. Pornography involves explicit sexual behavior designed to titillate and arouse.
There is also the matter of intention. Artists have long considered the human body to be among nature's most interesting -- and difficult to draw, paint, or sculpt -- creations. A nude painting or sculpture exists, in part, to allow the artist to study the body and to demonstrate his skill. Moreover, an artistic nude usually appears in the context of a story or at least represents a story. The purpose of pornography is, as I said, solely to trigger sexual excitement.
By the way, some works of art can be considered pornographic by some people, and some pornography can actually be artistic. Sometimes these matters are entirely subjective. As a judge once famously said -- and I'll have to paraphrase him -- "I may not be able to define obscenity, but I know it when I see it."