1) make a 500x200 jpg image of your car or whatever is to be your sig image.
2) upload it to a hosting service ( I use the free service @ Villagephotos.com)
3) go to the edit signature in the user CP and type in the following line:
[ img]http://www.yourImageURL.com[/img ] where "your image URL.com" is copied from your pic on the hosting site. (take out the spaces after the first bracket and before the last one. I had to include them to make the code show up in the post)
I use Paint Shop Pro for my photo editing and my sig is a 7 layer composite image.