If you are not sure your use of content would be covered by fair use, and you want to be sure you are working with only copyright free resources, there are many places and ways to search for copyright free content. Use the tabs on the left hand side to find copyright free website content, images, videos, and audio content.
As you search for creative works to use in your own projects, publications, or teaching materials, you may see Creative Commons licenses attached to these works. Creative Commons is a nonprofit organization that provides free licenses that allow the owners of creative works to share their content with the public on their terms. This nonprofit organization offers a number of licenses ranging from "Some Rights Reserved" to public domain.
CC attributions vary, therefore you must read the summary of the license to understand how to use the image, texts, videos, music etc.
“A copyright troll is a party (person or company) that enforces copyrights it owns for purposes of making money through litigation, in a manner considered unduly aggressive or opportunistic, generally without producing or licensing the works it owns for paid distribution. Critics object to the activity because they believe it does not encourage the production of creative works, but instead makes money through the inequities and unintended consequences of high statutory damages provisions in copyright laws intended to encourage creation of such works.” (From https://en.wikipedia.org/wiki/Copyright_troll)
Although copyright infringement can occur in any medium, when posting material on a digital platform, it is quite easy to use images, sound, or video found and copied from other online sites. However, use of these materials might be copyright infringements.
The creator of a work (e.g. an image or photograph, a video, a sound clip, etc.) often holds the exclusive right to reproduce, publish, adapt, perform or display the work. Anyone who utilizes the work without the creator’s/author’s permission would then be infringing on the creator's/author’s copyright.
And, copyright trolls may be looking for these infringements! They often use automated programs to search the Internet for postings of these unauthorized copies of copyrighted works. They then send out cease and desist letters when infringements are found. The letters often also demand settlement payments or threaten lawsuits. The copyright trolls can and do bring litigation in federal court seeking damages, which can be quite steep.
If it is necessary to use someone else’s copyrighted work, it is also necessary to obtain permission to do so. Getting permission can be done through copyright clearing houses such as https://www.gettyimages.com/, https://www.prsformusic.com/help/contact-us, https://www.ascap.com/ or the Copyright Clearance Center at http://www.copyright.com -- or directly through the creator, the creator's organization, or the publisher. This will often require the payment of licensing or royalty fees but these fees are usually much less than what a copyright infringement could cost.
For additional information on Copyright Trolls:
Sag, M.,& Haskell, J. (2018). Defense against the dark arts of copyright trolling. Iowa Law Review, 103(2). https://ilr.law.uiowa.edu/print/volume-103-issue-2/defense-against-the-dark-arts-of-copyright-trolling/