Under U.S. law, any "work" (a product of a person's creative work) is automatically copyrighted from the moment it is created.
Copyrighted
You need to seek permission from the work's creator to use it unless your purpose meets the criteria for Fair Use. You must attribute (cite) any copyrighted work you use.
Fair Use
Fair Use is a user's right to limited use of a copyrighted work without permission or payment under certain conditions. You must attribute (cite) any copyrighted work you use, even under Fair Use conditions. See: Downloading Images from the Internet.
Creative Commons Licenses
A CC license is a type of copyright that allows the creator of a work to specify the ways and conditions under which a work may be used without permission or payment. You must attribute (cite) any CC-licensed work that you use (except CC0). See: Creative Commons
Public Domain and Creative Commons Zero (CC0)
You may use works in the public domain or with a CC0 license for any purpose without permission. Although you have no legal obligation to cite a CC work, attribution is usually expected in academic environments. Check with your instructor. If you are unsure, cite it anyway.
An extensive collection of symbols (icons) in raster (PNG) or vector (SVG & PDF) formats in the Public Domain or under Creative Commons licence.How to cite icons.
All photos and videos on Pexels are free to use and modify. While attribution is not required, giving credit to the photographer or Pexels is always appreciated.
A guide (like this one) covering the basics of how to use copyrighted material with a Creative Commons license, and how to use Public Domain materials with no copyright. From Tammy Ravas at University of Montana.
Millions of media files free for you to use under a public domain, creative commons, or GNU Free Documentation licenses. This site makes it easy to download, use and cite media.
This indexes and links to museum-quality fine art on the Internet. Search by artist name, medium, artwork title, location or museum, subject, nationality, art movement or period, and for art by women.
A simple way to find images by keyword. Filter by size, type (face, photo, clip art, line drawing) and color. Use the Advanced Search for more targeted searching, and more options for specifying size, aspect ratio, file types, and license.