17 U.S. Code § 107 is the mostly relevant statutes that govern fair use. There is no hard boundary on what constitutes fair use and what does not. It's a multi-factor balancing test: Courts consider the non-exclusive factors in 17 U.S. Code § 107, with the constitutional demand borne in mind (the Intellectual Property Clause, "To promote the Progress of Science and useful Arts").
17 U.S. Code § 107(1), "the purpose and character of the use," is very relevant to some of your hypotheticals. The Supreme Court once discussed this factor:
The central purpose of this investigation is to see, in Justice Story's words, whether the new work merely "supersede[s] the objects" of the original creation, Folsom v. Marsh, supra, at 348; accord, Harper Row, supra, at 562 ("supplanting" the original), or instead adds something new, with a further purpose or different character, altering the first with new expression, meaning, or message; it asks, in other words, whether and to what extent the new work is "transformative." Leval 1111. Although such transformative use is not absolutely necessary for a finding of fair use, the goal of copyright, to promote science and the arts, is generally furthered by the creation of transformative works. Such works thus lie at the heart of the fair use doctrine's guarantee of breathing space within the confines of copyright ... and the more transformative the new work, the less will be the significance of other factors, like commercialism, that may weigh against a finding of fair use.
Campbell v. Acuff-Rose Music, Inc., 510 U.S. 569 (1994). When taking a picture of a table or a chair is likely transformative use, and facilitates spreading knowledge on these designs.
Another relevant factor is, 17 U.S. Code § 107(4), "the effect of the use upon the potential market for or value of the copyrighted work."
The purpose of copyright is to create incentives for creative effort. Even copying for noncommercial purposes may impair the copyright holder's ability to obtain the rewards that Congress intended him to have. But a use that has no demonstrable effect upon the potential market for, or the value of, the copyrighted work need not be prohibited in order to protect the author's incentive to create. The prohibition of such noncommercial uses would merely inhibit access to ideas without any countervailing benefit.
Sony Corp. v. Universal City Studios, Inc., 464 U.S. 417 (1984). For example, if you post a picture of a book, and comment: "This is the book I'm reading right now!" Very likely this won't harm the book's market, if not actually boost it. Even if the post criticizes the book, it is a fair use explicitly enumerated in 17 U.S. Code § 107 ("the fair use of a copyrighted work, including such use ... for purposes such as criticism ... is not an infringement of copyright.").