Car Demon has a wide variety of PHP constants that can be used to modify functionality.
Over time many of these constants will be turned into settings that can be adjusted on the settings page. In the meantime you can define these in your wp-config.php file right above where it says:
/* That's all, stop editing! Happy blogging. */
CD_CUSTOM_NO_PHOTO will allow you to define a custom “No Photo Found” image to be used if your vehicles don’t have a main photo.
To do this you would add the following to your wp-config.php file:
define( 'CD_CUSTOM_NO_PHOTO', 'http://example.com/images/no_photo.gif' );