The car thumbnail option doesn't actually make a separate picture file of your car(s) like a photo, it just updates the thumbnail preview in the garage for the car if it's missing/glitched.
If your cars are still missing thumbnails after you create a thumbnail, then there's most likely an issue with the car itself (this can happen if you mess with hex files)
Okay, your car should have a thumbnail then - unfortunately, if you want a picture to use as a thumbnail elsewhere, you'll have to take a photo of your veyron from a replay.