If just a problem with car pictures, why not using Wikimedia's ones? Each file in Wikimedia can be re-used due to it's license (Creative Commons By or By/SA, public domain, WTFPL...) and can even be integrated into a commercial work.

It's almost the same thing with Filckr's pictures, when provided with a Creative Commons license. You just need to pay attention to a possible NC (Non Commercial) mention in the CC License, with Filckr's pictures, if you intend to sell your work.
Flickr also shows pictures labeled as "U.S. Government works", but I'm not sure you can freely use them outside the USA.