The concept of the artist and art as a theme is illustrated as a transmutation where the artist leaves a piece of his soul in his work. This is shown in the way that Basil insists that he had put "too much of himself" in the picture of Dorian.
Basil does not want to show the picture at the beginning of the novel for fear that the admiration and feelings that he has for Dorian would have somehow manifested in the way that he portrayed Dorian in the painting. If this happened, then he would be confessing to a very homophobic Victorian society his true emotions (which would have constituted a crime).
He later changes his mind and confesses to Dorian openly
.. I cannot help feeling that it is a mistake to think that the passion one feels in creation is ever really shown in the work one creates. Art is always more abstract than we fancy. Form and colour tell us of form and colour—that is all. It often seems to me that art conceals the artist far more completely than it ever reveals him.
Basically, Basil agrees with Wilde's statement in the prologue of the novel that
All art is quite useless
which basically means that art has no moralizing job to do; it is not a transmitter of the soul, and it should not be used to educate, edify, teach us about love, or help us with anything other than show its beauty. Art is to be admired. Art for the sake of art. This being said, "L'Art pour l'art" is the main tenet of the Aesthetic movement of which Wilde was at center stage.
Aestheticism is defined as the enticement of the senses with beauty. We know that Lord Henry preaches in favor of this by inviting Dorian to adhere to all temptations and indulge every one of his senses. In Henry's personal canon of life, that is the chief responsibility of those born to the "New Hedonism" where all men deserve to seek and find pleasure in everything.
To cure the soul by means of the senses, and the senses by means of the soul." Yes, that was the secret. [..] There were opium dens where one could buy oblivion, dens of horror where the memory of old sins could be destroyed by the madness of sins that were new.
Dorian corrupted the idea of aestheticism and turned it into an aberrant form of hedonism; a gross overindulgence where he confused the want for beauty with seeking carnal pleasure, bringing down with him whoever got in his way. He ruined reputations, lost friends, and corrupted his soul in the process. Of this, Henry tells Dorian:
Live! Live the wonderful life that is in you! Let nothing be lost upon you. Be always searching for new sensations. Be afraid of nothing.... A new Hedonism—that is what our century wants.
We can all agree that Dorian goes overboard with this "life" that Henry proposes for him. Dorian literally drinks from the cup of each deadly sin under the guise of seeking new emotions. Thinly-veiled homoeroticism is at the top of his list.
All this leads to the battle between ethics versus aesthetics. This debate is rampant all over the novel, even at the prologue where Wild defends his controversial novel by explaining the tenet mentioned before of "Art for art's sake". In Wildean theory art and ethics have nothing to do with one another. Art is the result of creation, and creativity does not follow social rules the way that ethical behavior does. They do not mix.
No artist has ethical sympathies. An ethical sympathy in an artist is an unpardonable mannerism of style. No artist is ever morbid. The artist can express everything.
The aesthetic/artistic tone Lord Henry insists for Dorian to add to his (Dorian's) life entails that all ethical behavior is overrated and repudiated. As we know, Dorian took no time in engaging in Henry's suggestions to the point of corrupting whatever was left of his soul, which was manifesting supernaturally in the painting.