Unless explicity outlined in license agreement, you have to assume any image you find online is the property of someone else and it cannot be used without permission.
See Common Copyrigth Myths:
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The international "protocol" is actually a treaty called the Berne Convention to which the United States adhered March 1, 1989. Rather than new protocols, the treaty greatly simplified copyright protection by eliminating formalities such as copyright registration and notice that had been previously required under United States law. Copyright registration and notice are still very advantageous under US copyright law to enable enforcement and collection of enhanced damages.
This means that you can assume that if it is on the Internet it is protected by copyright, unless it is very, very old-typically before 1923.
Reproduction and use is typically not legal, since it violates the exclusive rights of a copyright owner under 17 USC 106, whether or not commercial. The fair use exceptions are defined in 17 USC 107, which does not use the term "non-commercial" but rather "nonprofit educational purposes." If you read 17 USC 107, you will understand that this is a very subjective standard that may vary from case to case. And so it is very important for you to consult an experienced copyright attorney who has addressed this issue repeatedly in the past and has a good feel for what the cases are currently holding to be "fair use".
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One of the biggest misconceptions is that a photograph can be used for any non-commercial purpose. That is fundamentally wrong. Non-commercial uses infringe no less than commercial uses unless such use is found to be fair use.
Further, it is rare that the fair use doctrine allows use of a photograph without consent of the owner of the copyright. Fair use is a defense to copyright infringement that allows a copyrighted work (or a portion of it) to be used in certain very limited circumstances. Courts consider many factors in a fair use analysis including the nature, purpose, scope and duration of the use, the portion of the copyrighted work used (i.e., a quote from a book rather than the entire book), and the impact of the use on the market value for the copyrighted work. Fair use most often applies where a copyrighted photograph is used for purposes of education, social commentary, news and journalism, academic analysis or parody. But you should not assume that every educational use is fair use, nor should you assume that every use for purposes of news reporting and journalism is fair use. Fair use will not excuse use of a newsworthy photograph if such use would diminish the economic value of the photograph to the copyright owner. Courts are especially reluctant to find that fair use applies to photographs because the entire photograph is often used, which inevitably diminishes the commercial value of the photograph.
Further, fair use is only a defense to a charge of infringement. Even if you have a strong fair use argument, you have to prove that argument in court, which can cost hundreds of thousands of dollars.
The bottom line is this--for every photograph you should presume that you need permission or a license from the copyright owner before you use it, republish it, share it, or otherwise profit from it.
I think Edwin hit this one on the head: these sites will have policies on their ToU. A fuller analysis of this general question could fill a book...
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