Saying James Dean is a style icon is kind of like saying Marilyn Monroe is a style icon. As in "No duh."
I mean, the dude is more iconic than any other icon. He's James Dean. Just like Marilyn, you don't have to see his movies to know who he is and admire his cool-guy look. (But see his movies. Especially "Rebel Without a Cause." It's a classic for a reason and James Dean is never James Dean-ier than he is in that movie.)
So James Dean is totally the insanely gorgeous boy smoking a cigarette in a leather jacket, but he also has a more laid-back kind of bookish style too. James Dean was a thinker! He was sensitive! He hung out with Elizabeth Taylor and she took naps and he read celebrity magazines.
He looked amazing in sneakers and glasses. He probably would look amazing in anything being that he was James Dean and all.
But, yes, the cigarette/jacket look is probably his best. You're a dreamy one, James Dean. You're definitely a dreamy one.