This sounds like a graphics issue, not a coding issue, because you linked to this:
<img src="http://content.screencast.com/users/karlvenus/folders/Default/media/63be76af-4ecc-42ae-b667-898454f3198a/128tower.png" border="0" />
Honestly, it doesn't look too hard - make a circle and a triangle.
For implementation, there is a Turret behavior and there are numerous tutorials on how to use particles in C2, although I'm pretty sure what you want is a turret projectile, not a particle ;)