Something reasonably well known, a full car shot. You can blank out what you may think is obvious but leaves a clue. Eco-biased if you like, a concept, prototype, classic, or every day car from a show. You can include more than one photo and a clue, maybe a period or a country or region or production.
Hey I just made this up, so the rules are kind of in flux
EDIT - you can use the photo albums in your user profile to upload the blanked images and link to that. Obviously miss out the car name from the pic - that may be too obvious...
EDIT2 - You can even include just an indicator or a headlight, if it THAT well known...