In the past, photo editing was time consuming and tedious. Anything beyond simply lightening or darkening a picture meant hours of painstaking work. Something as simple as painting away a feature involved creating an entirely new picture, with the object being replaced by tiny bits cut out of other parts of the picture. The extensive work involved in, say, painting out one of Stalin's former colleagues, probably took days.
The world of photo editing today is completely different. Paint programs like Photoshop make it easy to improve the features of a photograph, giving the photographer a bit more leeway with lighting and exposure. Unfortunately, they also make it very easy to change the photo, and present something that wasn't really there when the picture was taken.
Photojournalists have a responsibility to present facts, not fiction. Editing the picture to correct a color cast is not the same as changing a dull grey sky to a brilliant red sunset. Adding smoke, or multiplying the number of people in a scene, do not make the picture more "dramatic" or "more representative" of what happened--they lie to the viewer, in the same way that putting a celebrity's head onto another person's body is a lie.
At what point does the photographer cross the line from "improving" a picture to "improving upon" it? When he adds or subtracts elements that change the message or meaning of the picture. Adding or removing information, even by simply cropping out damage or blurring critical information, is the line that photojournalists must not cross.
Keep these facts in mind when editing photos. If a picture is "artwork," and not meant to be a scene of reality, then the artist is free to edit as he chooses. But a photojournalist is not an artist, and news photography is not supposed to be art.