Only the image matters:A post is 2D when it has a 2D image ONLY. A post is 3D when it has a 3D image ONLY.
i.e. a post about the real world is still "2D" so long as it has a 2D image attached. Conversely, a post about an anime, and that only discusses the anime is "3D" if a 3D image is attached.
Only the content matters:A post is 2D when it talks about 2D things (Anime, manga, video games, etc.). A post is 3D when it talks about 3D things (IRL people, news, outdoors, animals, etc.)
i.e. a post can have a 2D image, but if it the post is about the real world, it is a "3D post." Conversely, a post can have a 3D image, but if the post is about an anime, then it is a "2D post." The classic example is animal videos are typically considered acceptable on "2D" boards.
Both matter, but the image matters more:A post can be either 2D or 3D depending on the content, but the image matters more.
i.e. a thread about a seiyuu from a particular anime with a 2D image is a 2D thread more often than not, HOWEVER, a thread about a seiyuu from a particular anime with a picture
of the seiyuu is a 3D thread more often than not.
Both matter, but the content matters more:A post can be either 2D or 3D depending on the image, but the content matters more.
i.e. a thread about a particular anime with a picture of a seiyuu is a 2D thread more often than not. A thread about a seiyuu with a screenshot from the anime is a 3D thread more often than not.