I've ended up developing a number of different car images for use on the PBeM.
For the year specific cars, I was a bit limited by which cars I could find straight, top-down sketches of (which is rarer then you might think). So, for every year I ended up using the same manufactorer car body for multiple cars in order to fill out a possible 12 car field. I tended to dupe manufactorers that were most successful.
I also tried to make the colors of cars match up to the liveries of those cars that year, but mostly wanted to make sure I had a different color for each car so that does not always work.