Added even more elements, including the overgrown old tire and a gas mask under the car. Also added a small radioactive sticker on the large cargo container.

I added in some shading and light, as well as the tree and a few more adjustment layers so that everything fit in even better.

I added more elements to make the scene more apocalyptic, and adjusted their levels and even colour slightly to add a yellow tinge to make them fit in better.

I began with this image I found on flickr (within creative commons of course!)