Use of this work which I have created is allowed under the terms of the CC-BY-SA license (see below). Credit must be given to "Wikimedia Commons." You can contact me if you would like to negotiate other licensing terms (which I am generally happy to do). Those who wish to use this in the course of teaching may do so without any limitations. --Fastfission
This file is licensed under the Creative CommonsAttribution ShareAlike 2.5 License. In short: you are free to share and make derivative works of the file under the conditions that you appropriately attribute it, and that you distribute it only under a license identical to this one. Official license