Reading Images

In the article "Reading Images" by Gunther Kress and Theo van Leeuwen, the authors break down the composition of photographs, in order to depict the meaning behind the images.

Most notably among the various compositions, in my opinion, was Kress and van Leeuwen's explanation of why compositions matter.  The authors state that composition as it is selected by photographers and layout editors, develops the way that readers view the image (and often its corresponding article).

The type of photograph that I personally find the most striking is the "fly on the wall" photograph.  The authors explain this type of image as one that the subjects are unaware that someone is photographing them, unaware that this event or experience or time in their life is being recorded.  One example that immediately came to my mind was the following:

This photograph was taken by a "fly on the wall" and yet it had a tremendous impact result from it.  The photograph was taken by Wayne Tilcock of The Enterprise and was captioned, "University of California, Davis Police Lt. John Pike uses pepper spray Friday to move peacefully Occupy UC Davis protesters while blocking their exit from the school's quad in Davis, CA.  Pike is a retired US Marine sergeant twice honored for his police work on campus."

The image's photographer may not have known exactly what the media audience's reaction would be to it, but he still had a purpose in mind when he took the photo.  It was to very clearly make a statement about the almost "assailant" role the police officer played for the "victims" or students.

My questions for discussion are though, do all photographs have this same power?  Who decides the impact that a photograph will have?  Is this another case of the infamous "CNN Effect"?  How is the CNN Effect determined with regard to photographs?  Do all photographs have credibility if we know that photographers take certain pictures to represent certain perspectives?