Trace the changes that occur in the portrait over time. How does Wilde make the reader feel that the portrait is in fact magical and not just a figment of Dorian Gray’s imagination? Why is it important that Wilde makes this distinction?

Dorian's first awareness of the portrait's change occurs in Chapter 7 after he returns from the theatre, having just told Sibyl Vane that their relationship is off and he wants nothing more to do with her. At first, in the "dim, arrested" light of the curtained room he thinks only that "the face appeared to him to be a little changed," showing "a touch of cruelty in the mouth." One gets the feeling it might simply be an optical illusion at this point, until Dorian looks at the portrait after raising the blind. In the direct sunlight, and especially after comparing the picture with his own face in a mirror, he sees that

There were no signs of any change when he looked into the actual painting, and yet there was no doubt that the whole expression had altered. It was not a mere fancy of his own. The thing was horribly apparent.

Yet at this point, and later, Dorian himself is the only one who has seen the portrait change. When he goes to bed in the early morning, he has somehow still convinced himself that the altering of the painting was just an illusion "wrought on his troubled senses."
When he looks at the painting again after waking from a long sleep, there finally can be no doubt that it has changed. At first Dorian speculates that there might be some "chemical affinity" between the atoms of the paint and his own soul. He knows that in rejecting Sibyl his cruelty was horrible, and so he resolves to go back to her, not knowing yet that she has committed suicide. When Lord Henry finally gives him the news of it, this seems to clinch not only Dorian's fate but that of the portrait.
Yet there is still no positive proof that it's not a hallucination on Dorian's part, since no one else sees the picture until he finally shows it to Basil the night he murders him. By this time it has changed much more, and is barely recognizable to the man who painted it. Dorian stabs Basil to death in revenge for what he believes the painter has done to him, as if it has been the portrait controlling Dorian all along.
The close of the novel, where Dorian stabs the portrait but the knife is then found by the servants in the body of a loathsome, withered old man (while the portrait is in perfect condition showing Dorian as a young man), is Wilde's expression of the illusion vs. reality theme. He seems to be asking, is "real" life more real than art, or is art the "true" reality? The answer is anybody's guess.