Sorry, been off-line for a couple of days.
Some specs are here:
http://create.tomtom.com/manuals/create-your-own-content/
Basics for the final icon are:
24 bit .bmp format, size 80x80, Background full red (255,0,0)
The way I find works best is to work with the full sized image first and to cut the car out using whatever "paint" program you have, by outlining it reasonably carefully.
then create a new blank 80x80 image with a red background and paste the cut-out car into it, resizing it to just fit in the box.