In The Picture of Dorian Gray, how does Dorian get the idea of having the portrait age instead of him?

Dorian gets the idea from Lord Henry Wotton, who talks about a philosophy of life in which morality is set aside in the pursuit of fulfillment through the enjoyment of sensuality and passion.  Dorian, who is enamored by his own youth and beauty, decides that he would give anything, even his own soul if, instead of growing old, he could retain his good looks and have a portrait of himself show the ravages of aging instead.

The portrait to which he is referring has been painted by his friend Basil Hallward, who has envisioned a new form of art in which soul and body are linked in perfect harmony.  Dorian first hears Lord Henry expound on his ideas, and makes his fatal wish, at Basil's house when the artist has just completed the picture.  Fittingly then, as Dorian pursues his hedonistic way through life, the face Basil has rendered on canvas really does become more and more sordid, in reflection not only of age but of the dissipation of its subject's soul.