I imagine WB owns the rights to the letters/envelope designs, and the photos of them, whoever took them, or whoever is the licensor (copyright holder) of those photos, would own the rights for the usage of the photos.
Transfer of rights for the photos would either be specifically licensed for a particular use/duration or bought out exclusively. In this case they're product shots from the prop site, (hence the watermarks). Buying something doesn't mean that all reproductions, like photos taken of the item belong to the owner of the original item, unless transfer of rights of said photos were specifically stated in a contract otherwise.
Anyone is free to repost those images as they wish, as they have been introduced into the public online domain, and ideally crediting the original source or artist of the images. Usually there is no issue as long as the images are not being used to generate revenue. This applies to all sorts of visual media. Selling those photos (which i doubt anyone would do) would normally be a violation of copyright.
Copyrights are a long and dense complicated matter.
@eviltig: The chrome looks Awesome! I like it a lot more than the gray, and it matches the rest of the badges on your truck.