I think you would have to make a 4th powerup. Then in the ship class, make it so that if it gets that powerup, every frame it does "shootTimer += 1" again, so that it fires twice as fast, then make a seperate timer so that it ends after say 5 seconds, so at 30 fps, thats if(blah > 150)
{code to stop rapid fire here}
It's not an image, you have to open the .fla file, then int he library, highlight "Explosion", control-c, then go back to your flash file, then press control-v and it will be in your library.