Here some useful GIMP tutorials for videogames:

http://www.wingimp.org/tutorial/fire/fire.html

http://www.wingimp.org/tutorial/fire2/fire2.html

http://www.wingimp.org/tutorial/reflection/reflection.html

http://www.wingimp.org/tutorial/rusty_obj/rusty_objects.html

http://www.wingimp.org/tutorial/shot/shot.html

http://www.wingimp.org/tutorial/reflection3/reflection.html

http://www.arcanefractals.com/tutorial_part1.shtml

http://gug.sunsite.dk/tutorials/tomcat9/

Btw, you can upload the TGA wwhere you cant get rid of the background, then I will tell you whats wrong.

I have to say game looks quite good. I watched that 43mb or something trailer. Just some sprites are awful, like the yellow one when you shoot. I hope you will make better ones.