Images made available under a Creative Commons licence, or images which are in the public domain, can be used for educational purposes without infringing copyright.
Whilst most of the collections below contain only their own images, it is not uncommon for some sites (eg Stocksnap, Pixabay) to include promotional content from other companies on their pages. Please ensure that if you are selecting images from these sites, they are not from another source and they are available under a Creative Commons licence.
It is good practice (and usually a licence requirement) to include an attribution with any image you use. Check a site's/image's terms and conditions to see what details are required in the attribution.
General image collections
Classical art public domain paintings, drawings, illustrations and posters.
Use the 'Any license' dropdown on the results page to select the 'All creative commons' option ; You will still need to check the rights allowed - this is indicated under the photo (the 'Some rights reserved' link provides details).
Wikimedia Commons (includes images, sounds, videos)
Conditions of use - https://commons.wikimedia.org/wiki/Commons:Reusing_content_outside_Wikimedia
Discipline focused collections
Cell Image Library
"...a repository for images and movies of cells from a variety of organisms. It demonstrates cellular architecture and functions with high quality images, videos, and animations"
Conditions of use: Not all images will be available for re-use: check the copyright status on the image. http://cellimagelibrary.org/pages/license
The world's largest database of digitised plant specimens; includes photographs, botanical illustrations, diaries, reference works and correspondences
Conditions of use: https://about.jstor.org/terms
NASA Image Galleries
Conditions of use: https://www.nasa.gov/multimedia/guidelines/index.html
National Institute of General Medical Sciences (NIGMS) Image and Video Gallery
Conditions of use: Licence conditions stated on the home page; see 'View Entry' on the individual image for attribution details
Public Health Image Library (PHIL)
Collection of health/medical images from the CDC.
Conditions of use: check the 'Copyright restrictions' note on the image details
UK health-focused museum and library's digital collection. Content reflects the nature of the collection (a large portion is historical in nature).
Conditions of use: Unless otherwise noted, content is available under a creative commons licence. Licence and attribution details for individual images displays when you download the image.
California State University IMAGE Project's database of images; global in coverage
Conditions of use: http://worldimages.sjsu.edu/kiosk/conditions_of_use.htm
Australian Libraries - Digital Collections