Labeling Should Be Banned
Opponents claim that this would require banning, labeling, or warnings on a multitude of advertising materials. For example the roads used in car advertising are never as serene as they appear. If labeling is required for all digitally altered images, it would extend across a wide range of graphic imagery and require costly and time-consuming measures to enforce it.
If you add something that wasn't there in the original scene, you've crossed the line from a documentary art form into a fictional one. This may or may not be OK, depending on what your purpose is. If your purpose is to portray a scene as truthful, then it's not OK. If your purpose was to create fiction, or "art", then it is OK. You just have to be up front and tell the viewer what you are doing in either case.
In some situations it would be unethical not to digitally alter the content of a photograph, such as when a photo definitely records something incorrectly, such as red eye. The red eye would never have been there if we didn't change the original scene by adding the