Are you trying to cut out the background of the car first?
Fireworks is a vector web graphics program. There is some bitmap (image) manipulation that you can do, such as mask items, use the lasso, magic wand, clone tool, blur tool and other image manipulation items, but it isn't meant for pure image manipulation. What you are describing, sounds like it might be a better job for Photoshop or Elements instead of Fireworks. Sorry. Fireworks is amazing at website design, UX design, mobile design.