You know how some titles are little mysteries in themselves, and how they can make you wonder, "Man, what was that author thinking?" Well, rest easy, because this is not one of those titles.
The Picture of Dorian Gray refers quite straightforwardly to two portraits: first of all, the very literal picture of Dorian painted by Basil Hallward, and secondly, the literary "picture" Wilde creates in the novel.
Both of these works of art show us what the so-called "real" world can't see—the truth of Dorian's soul. The painting itself is at the center of the whole novel; while Dorian's physical beauty remains untouched, the Dorian captured in the painting changes horribly to reflect the corruption of his soul. Just as this picture shows viewers (well, there's really only one viewer—Dorian himself) the true nature of its subject, so too does Wilde's novel reveal Dorian's increasingly evil inner self to us, the readers.