Lesson Plans

First, just because one does create a cease and desist letter, does not necessarily actually mean anything. This is often just a scare tactic. In this situation, this may be the case. I believe that this is fair use. First, the material is being used for non-profit educational use. When claiming fair use, one looks at the purpose of the material. Under the Best Practices in Fair Use for Educators, one can create and share curriculum materials that include copyrighted materials. The teacher could even sell this material because it is for curriculum use.  The next question is the question of transformative.
I would argue that this is a transformative use of copyrighted materials because the purpose of the graphics has been transformed thus making it appropriate for one to claim fair use.
Finally, I would argue that the small group of educators using the graphics would not affect the market value for the graphics.