Images on the open web are subject to copyright law in the same manner as any other creative work; there is no guarantee that an image is legally available for re-use just because it is freely accessible on the web.
That said, there are many cases in which copyright law permits re-use:
1. The image is in the public domain.
2. The image is available under a Creative Commons (CC) license.
3. The image is otherwise made available for re-use by the content provider.
4. The image is copyrighted, but re-use qualifies as Fair Use.
5. You have permission from the copyright owner. For more information, see Getting Permission.
Using public domain or open-licensed images can be a great way to avoid the hassles of getting permission. These sites offer millions of such images. However, not every image included in these sources is guaranteed to be freely usable for every purpose -- be sure to review the copyright information for the particular images you select.
Here are several excellent sources for images in the public domain and creative commons-licensed content:
Flickr Advanced Search - Under Any License, select “all creative commons”
Google Advanced Image Search - Use the “Usage Rights” field to limit by license type.
Cleveland Museum of Art - Open access image search for CC0-licensed images
J. Paul Getty Search Gateway - open content (public domain or no known rights) images available for re-use marked with "Download"
Images of Empowerment - All photographs available under a CC BY NC 4.0
Library of Congress: Prints & Photographs Online Catalog - Photographs, prints, drawings, posters, and architectural drawings, and more.
NGA Images - Public domain artworks from the collections of the National Gallery of Art.
NYPL Digital Gallery - Illuminated manuscripts, historical maps, vintage posters, rare prints, photographs, and more, from the New York Public Library.
Open F|S - Over 40,000 Asian art images from the Smithsonian's Freer Gallery of Art and Arthur M. Sackler Gallery available for non-commercial use.
Unsplash - High quality images freely licensed for any use.
Wellcome Images - All images are made available under Creative Commons licenses.
Wikipedia Public Domain Images - List of public domain image sources on the web.
It can be difficult to determine the creator of a web image. To make giving credit easy, look for images that give you enough information to attribute them. Use the citation style you plan to use for the rest of your work, or use a general format as in the examples below.
Where should you give credit? Give credit underneath the image, at the bottom of the page, or in a credits section (e.g the last slide in a PowerPoint).
At minimum, do your best to:
1. Link to back to the original work
2. Give credit to the image creator
3. Follow attribution instructions provided by the source
Generic Image Credit Format:
Title by A. Creator is licensed under [license type].