I use two different programs.
I use adobe photoshop to cut out and paste the cars onto a background as a moveable layer, flatten the image and copy to clipboard, the I use painter5 to generate the ani gif by pasting the frames into each slide, moving the car a little each time. easy.
both programs are over 5 years old.
there are better ones today but i dont keep up with it. Anfy is a good program for making java appletts, very cool effects and easy and freeware www.anfy.com