Seems that my version of Camtasia can't handle the 'key hook' when it is (semi-)transparent.
So I don't know whether it works with a picture with alpha transparency, but a picture without alpha channel might be sufficient.

Thank you for looking into this.

http://www.puppenheim.org/
http://www.wuerfels-welt.de
Old Game Tutorial: http://www.puppenheim.org/MITutorial.zip
Free Game Editor: http://puppenheim.org/AlienalsWeihnachtsmann.zip
http://www.puppenheim.org/blender_starting_main_settings.txt