Courts have held that posting a work (text, picture, sound, video, etc.) on a publicly accessible website is a form of publishing. If you publish something without asserting your copyright, then you are placing that work into the public domain. Obviously, if the picture has a copyright notice right on it, then the copyright has been asserted. But, many websites also have an umbrella copyright notice on the first page. It's much like a printed book which doesn't have a copyright notice on every page, but just one of the first that then covers the entire book. These umbrella notcies have to be respected as well. If the artist wishes to retain his copyright, that is his privilege and you must respect it. You must respect it legally, yes, but also if you wish others to respect your rights, copy and otherwise. Curiously, works published after March 1, 1989 doesn't necessarily need a copyright notice on it. Determing what is copyrighted and what is public domain got really complicated legally after March 1, 1989.
Now, a copyright doesn't lock a work up. It doesn't mean that the work can't be reused. Now we enter the worlds of "fair use," and "derivative work."
Fair use says that you can use a quotation from a copyrighted work without the author's permission, even against the author's will, for discussion or criticism of that work as long as you quote only a small fraction of that work, as long as the quotation comprises only a fraction of your work, as long as your quotation is for the purposes of discussing the original work, and as long as you make the original author clear, cite the original source, and mention that the quotation is copyrighted.
Allow me to quote from the US Federal Code, Title 17, Chapter 1, Section 107:
Sec. 107. Limitations on exclusive rights: Fair use
Notwithstanding the provisions of sections 106 and 106A, the fair use of a copyrighted work, including such use by reproduction in copies or phonorecords or by any other means specified by that section, for purposes such as criticism, comment, news reporting, teaching (including multiple copies for classroom use), scholarship, or research, is not an infringement of copyright. In determining whether the use made of a work in any particular case is a fair use the factors to be considered shall include -
(1) the purpose and character of the use, including whether such use is of a commercial nature or is for nonprofit educational purposes;
(2) the nature of the copyrighted work;
(3) the amount and substantiality of the portion used in relation to the copyrighted work as a whole; and
(4) the effect of the use upon the potential market for or value of the copyrighted work. The fact that a work is unpublished shall not itself bar a finding of fair use if such finding is made upon consideration of all the above factors.
(BTW, the US Federal Code is considered public domain for the purposes of copyright law and can be quoted freely).
There's a nice discussion of Fair Use on
this website.
A "derivative work" is when you use someone else's work as a basis or as a part of a larger work that you, yourself, have done. It's a completely different subject.
Now, here's an interesting question: If Benchmade publishes a picture of a knife on their website and I put that picture into a post here on the forum for the purposes of discussing the knife, that's fair use clearly (provided they cite the source, etc.). The thread is discussing and critcizing the knife. The real purpose of my thread is that discussion which is a new work itself.
If I, as a knife collector and amateur photographer, buy that knife and take a new and unique picture of it and post that picture on my website as art, and someone else finds it and posts it in a thread discussing and criticizing the picture, my photograhic technique, etc., that is also fair use (again, providing that the source is cited, etc.).
BUT, if someone else takes my picture and uses it in a thread to discuss the knife, that may not be fair use (it could be a derivative work, though).
