This image solely had a huge impact on Obama's campaign. McCain as himself probably helped, but that's beside the point. Done by Shepherd Fairey, the image was originally based on a photograph by Mannie Garcia. He was later sued by the Associated press for copyright infringement.
It was originally printed as a grassroots poster to spread the word of Obama's campaign. It later took a life of it's own, being spread in just about every medium possible. Later, on October 16, 2009, Shepherd Fairey admitted that he had taken copyrighted work and defaced it. In a court aggreement, he agreed to work with the associated press on further projects.