There was actually a pretty interesting bit of marketing to that image. Apparently, the site
www.ibelieveinharveydenttoo.com was set up as a campaign portal for the character of Harvey Dent. If you hung out on the site long enough, the site would be hacked and the picture of Dent would slowly morph to the Joker pic. If you go there now, the site appears to be down - until you highlight all the black space.