National Review, a periodical that Fox News viewers think of as the magazine their literate friends read, published this cover in 2012 showing Obama supporters waving blue signs that said "ABORTION." The signs originally said "FORWARD" before they were altered without attribution by the editors.
Wired has a gallery of other infamously altered photographs. They're part of a larger collection at the Bronx Documentary Center, called Altered Images.
In the original photo, the signs read, “FORWARD.” National Review, a conservative news magazine, changed each sign to read, “ABORTION.” They credited the photographer and Reuters news agency, but failed to indicate the photo was altered.
National Review publisher Jack Fowler responded to the issue, acknowledging that the image “in both the print and various digital editions, was altered by National Review. It is not the original photograph as provided by Reuters/Newscom, and therefore should not have been attributed to this organization, nor attributed to the photographer.”