Dorthea Lange's "Migrant Mother"

Many of you are familiar with Dorthea Lange's "Migrant Mother" photograph.  The photograph depicts Florence Thompson, age 32, with three of her seven children. As destitute pea pickers in Nipomo, California she came to epitomize the plight of "Okies" and displaced farmers during the height of the dust bowl in the 1930's. This series depicts multiple versions of the scene that would become one of the most iconic American images of the Great Depression. It should be noted that the negative was retouched in the 1930's to erase the thumb holding a tend pole in the lower right hand corner. Clearly this is an early example of image manipulation. 

Fast forward eighty years and we live in a world where images can be altered easily and convincingly. The old adage that "photographs don't lie" is undoubtedly an obsolete concept. And yet, viewers expect that news and documentary photography provide a neutral and honest representation of events. So when is it ethically unacceptable to alter an image? Is photographic veracity an antiquated concept or a fundamental line that should never be crossed? 

Here's another famous and controversial image by Robert Capa depicting the death of a Republican soldier during the Spanish Civil War. If this were a staged photograph would it have the same impact?