That said, there are many cases in which copyright law permits re-use:
1. The image is a public domain work. Generally, anything published in the U.S. before 1923 is in the public domain. For more info, check out the Peter Hirtle's Copyright Term and the Public Domain.
2. The image is available under a Creative Commons license. CC images are labeled as such. When using a CC image, be sure to provide proper attribution to the source.
3. The image is otherwise made available for re-use by the content provider. Some websites permit you to re-use their images on your own website, as long as certain conditions are met (e.g. noncommercial use only). In these cases, you can find out whether re-use is permitted by looking at the website’s Terms & Conditions.
4. The image is copyrighted, but re-use qualifies as Fair Use. In the context of using images on a website, you have a stronger Fair Use argument if you are directly commenting on or critiquing the image, or if you are using the image in a way that is transformative.
5. You have permission from the copyright owner.
Legally Using Images. Article by Leslie Ellen Harris Legally Using Images
Images from these databases may be used by Marshfield Clinic Health System employees for internal education and patient care. Please use appropriate citation.
Here are several excellent sources for images in the public domain and creative commons-licensed content:
Medical Gallery of Blausen Medical 2014 - Excellent source of labeled, anatomical illustrations - free to use through Creative Commons
CDC Public Health Image Library (PHIL) - Free to use images from a variety of subject areas
National Cancer Institute Visuals Online - The collection includes biomedical, science, and patient-care related illustrations and images. Some images do require permission.
NIH Images and B-roll: Please visit the NIH Flickr site(link is external) to view free-to-use NIH health research images, b-roll, infographics, and videos.
Here are several excellent sources for images in the public domain and creative commons-licensed content:
Burst from Shopify: Free stock photos
College Art Association Image Sources - Links to free, restricted and unrestricted, image banks.
Google Advanced Image Search - Use the “Usage Rights” field to limit by license type.
Library of Congress: American Memory - A free “digital record of American history and creativity.”
Library of Congress: Prints & Photographs Online Catalog - Photographs, prints, drawings, posters, and architectural drawings, and more.
NeedPix: These image are freely available under the Creative Commons Zero or CC0 Creative Commons license. They can be freely used by anyone for any purpose, whether commercial or non-commercial. Attribution is optional.
NGA Images: Public domain artworks from the collections of the National Gallery of Art.
Open Clip Art: Clip art that is free to use for any purpose. Requires setting up an account. Attribution: Creative Commons Zero 1.0 License
Openverse: Creative Commons Officially Launches a Search Engine That Indexes 300+ Million Public Domain Images
Notable collections: US Fish & Wildlife Service National Digital Library, National Park Service Digital Image Archives, and Smithsonian Images.
Smithsonian Open Access: You can download, share, and reuse millions of the Smithsonian’s images—right now, without asking.
Wellcome Images: All images are made available under Creative Commons licenses.
Wikimedia Commons: Browse or search for freely reusable images, free media resources / Photography.
Image credits: Public domain logo by Enoch Pratt Free Library and Creative Commons logo