I did this for a long time in photoshop. . . it is called photo chopping.
Check out some of these websites if you want:
http://www.digimods.co.ukhttp://www.photoshopjunkie.com/http://www.photo-chop.com/p.s. I suggest you cut back on some of your companies. . .And people wonder why GM is having problems 
 - Murphy Motors - Regular Main Company (like Chevy)  - Murphy Recreational and Equipment - Lawnmowers and ATVs etc. < and engines plus marine and outboards (like Honda) Honda/Yamaha (similar to any motorcycle company really) 
- Lujo Motors - Luxury Cars (like Cadillac)  Caddy/Mercedes 
- I-car - Regular car company (like Saturn)  maybe, I would go for a theme. . .maybe youth/entry level (like scion) or hybrid/fuel economy directed  
- Silver Star - Near-Luxury Car company (Like Oldsmobile)  They stopped Oldsmobile because it was costing GM too much money 
- Golden Star - Mid-Luxury Car company (Like Buick)  Buick is possibly next on GM's axe list (or should be). A mid level car would be a nice idea (Like VW in the US as compaired to luxury audi and super luxury Bentley) 
- MRC - Sports Car company (Like Porsche or Lamborghini)  - Alero - Regular car company (like Pontiac)  if you can differentiate it from the other companys 
[color=#ff0033]- Hama