The three central characters in this novel--Dorian Gray, Basil Hallward and Lord Henry Wotton--each have their own specific role in this story, and the type of character that each one is links strongly to this role. The novel is, at heart, a story of temptation, and various critics have likened the roles of Basil Hallward and Lord Henry Wotton to the angel and the devil fighting over the soul of a man. In this case, the man is Dorian Gray, and it is clear from the novel that Lord Henry Wotton represents the more hedonistic forces of evil and Basil Hallward represents the good. Note the way that Basil tries to keep Dorian away from Henry and argues against the hedonistic path he is treading. As such, these two central characters are clearly round and static in the way that they do not change their moral position in the novel.
However, Dorian Gray clearly changes in the course of the novel as he follows Lord Henry's hedonistic creed and makes the most of the magical portait that allows him to do anything and to bear no signs of depravation or degredation on his own soul. Note the following quote from Chapter 11:
Yes: there was to be, as Lord Henry had prophesied, a new Hedonism that was to re-create life, and to save it from that harsh, uncomely puritanism that is having, in our own day, its curious revival.
Dorian is therefore very clearly a dynamic character in the way that he changes as he adopts this "new Hedonism" that Lord Henry espouses, and which he is able to make the most of thanks to his magical portait. In addition, the psychological complexity of his character renders him a round character who is fully developed. There is very little that is two-dimensional about him.