Photo Tampering in History
an interesting academic website about the use of photo tampering, especially in news reporting, over the years... This would be good to share with high school students in a photoshop class, included in a discussion of digitally composited images and ethics.
To help contend with the implications of this tampering, we have developed a series of tools for detecting traces of tampering in digital images ...
http://www.cs.dartmouth.edu/farid/research/digitaltampering/