I've just been messing with various editing programs for awhile, teaching myself. This is how I do it:
1. copy the picture and make a new one2. use the erase tool and remove everything under the car by hand, so that it is transparent3. Copy the edited picture on top of the old one, allign it and check for errors4. fix any errors, merge it, and save
Basically, I copy everything above the bottom of the car's chasis over the old picture, then just select the detail in the picture.
Thats all
I use paintshop pro 7
For Rex's car, the guy standing in front made it abit tricky. I didn't use him in the seconc picture, and I left him in the original picture. When I copied the edited picture on top of the old one there he would remain the same height, but when I did that part of the first picture was showing. So I made his arm longer and covered it up

You can see where I edited the arm if you look hard enough. Also, you can tell that he has a longer than normal arm
