It's already happened. I've seen two photos so far (one of which was linked to in thread here on Mudcat) that somebody "PhotoShopped." The one some Mudcatter linked to showed a young John Kerry sitting next to Jane Fonda at an anti-war rally. He was there, but he was sitting several rows back, not beside her. Someone was trying to make a political point, and with modern technology, there is more than one way to tell a convincing lie. Another one showed Washington State Senator Patty Murray (shortly after she delivered a hard-charging speech in opposition to Congress giving Bush free war powers) playing a guitar (D-Model) with Saddam and a group of other Middle-East baddies standing behind her, labeled "Taliban Patty," but it was so crudely done it wouldn't have fooled anyone who didn't want to be fooled (the head was too big for the body and nobody but Quasimodo could hold his or her neck at quite that angle).
I also heard of a case a few years ago (but still recent enough for "PhotoShopping") where someone used a photo showing a man carrying a briefcase as he walked out of a building as evidence in what I vaguely recall was a spy case (can't recall the details at all, but someone else here may). The man accused said that he remembered the occasion in the photo very well, but he absolutely was not carrying the accusatory briefcase. He insisted that the photo had been altered, and that had he wanted to get the contents of the briefcase out of the building, he wouldn't have been stupid enough to carry it out in the briefcase in plain sight. As I recall, experts did manage to establish that it had been altered, but said the job was darn close to undetectable.