Mark Hancock is a photojournalist who spends an incredible amount of time on his blog - with research, trends and explanations of his work.
I have heard about various cases of image manipulation in news outlets over the years, and always wondered what these people were thinking. In the post linked to below, Hancock has detailed the specifics of multiple cases - going all the way back to Mathew Brady in the 1860s.
I encourage you to read through it - it is long, but very thorough and definately worth the time. I will be sharing this entry with my journalism students this fall.
PhotoJournalism: Image manipulation is a global problem