The problem was probably that transparency was set in MED: Object -> Manage Skins -> Skin Settings -> Texture -> Texture Format -> 24bit (36bit means transparency)

It was not the problem that it has been a png.

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