I'm not sure labeling would help any.
Here is the problem. In a situation like the one we currently have, a manipulated photo can be used to manipulate the public. Nothing new here as the Soviets did it, the Germans did, the U.S. did it.
The question is, is this allowable in a nation with freedom of the press? Many say that ethically the press takes on a huge obligation for accepting the freedom that it has. That in exchange for freedom, they must report the unbiased truth.
Lately this seems have been ignored. For example Fox News used its news ticker to spread Michael Moore jokes! Now excuse me, but that is just WRONG. I took journalism in college. That would have gotten ANYONE an F during their practicum! And a serious chewing out by Tomas Guillen (a very respected prof at my college and a Pulitzer finalist). Likewise "embedded" journalists would be verboten.
I'm not sure what the solution is though. People will rarely read the fine print to see if an image is manipulated or not. They will just assume it is real. Which is the danger. Imagine if they had Photoshop and the level of censorship they have now in Vietnam.
"Bombing of Cambodia? What bombing?"