It is genuine, it's by Warners.

I really like the image. It's a dark, grisly version of the Joker that really fits in with the Batman Begins style that Nolan crafted. Get Ledger all hyped up and smiley, throwing acid at Harvey Dent (Aaron Eckhart, as seen in the picture), and it should really work (and provide us with a villain for film number 3 ).

Hence I only classed it as a mild spoiler, since it doesn't show us anything we don't already know about the film - it's just showing it differently.