Images on the web
There are sites that can help you find copyright cleared images. For example http://search.creativecommons.org/ allows you to search across a collection of image providers. Note in the search results you should click through to the licence to check what the permitted usage is.
Other sites that provide images you can use in teaching
Wellcome images http://wellcomeimages.org/ Images from the Wellcome Collection, covering medicine, social history and contemporary healthcare.
VADS Online resource for visual arts http://www.vads.ac.uk/collections/index.html
JISC Media Hub http://jiscmediahub.ac.uk/
Courtauld Institute of Art http://www.artandarchitecture.org.uk/about/courtauld.html
World Register of Marine Species http://www.marinespecies.org/photogallery.php As well as an authoritative and comprehensive list of names of marine organisms, the World Register of Marine Species (WoRMS) provides images of marine life. Images are by default open-access under the terms of the CC BY-NC-SA license, unless stated otherwise.
Geograph Britain and Ireland http://www.geograph.org.uk/ The Geograph Britain and Ireland project aims to collect geographically representative photographs and information for every square kilometer of Great Britain and Ireland under Creative Commons licence.
Anatomy and Physiology Image Bank http://cbs.umn.edu/jensen/image-bank This collection of images is from The Sourcebook of Medical Illustration (The Parthenon Publishing Group, P. Cull, ed., 1989). The images are “copyright-free as long as they are used for educational purposes.”
Science Fair Project Images https://www.thoughtco.com/science-fair-project-picture-gallery-4123138 Collection of 74 public domain images. Please cite the source of the picture
Wikimedia Commons https://commons.wikimedia.org/wiki/Main_Page Wikimedia Commons contains over 55 million freely usable media files under a variety of licences
History of Medicine https://www.nlm.nih.gov/hmd/ihm/ Images from the History of Medicine (IHM) are from the historical collections of the U.S. National Library of Medicine. Here you’ll find image files of a wide variety of visual media including fine art, photographs, engravings, and posters that illustrate the social and historical aspects of medicine dating from the 15th to 21st century. Unless otherwise indicated, all content is in the public domain
Public Health Image Library https://phil.cdc.gov/default.aspx A catalogue of images from the center for Disease Control. Images are in the public domain and free of any copyright restriction. Please credit the image.
Image Gallery Gateway https://public.ornl.gov/site/gallery/default.cfm Images from the Genomic Science program, the Human Genome Project and Microbial Genomics. Almost all the images on these pages are original graphics created by the U.S. Department of Energy Genomic Science program's Biological and Environmental Research Information System (BERIS). You will recognize BERIS images by their credit lines. Permission to use these graphics is not needed, but please credit the U.S. Department of Energy Genomic Science program and the website https://genomicscience.energy.gov. All other images were provided by third parties and not created by the U.S. Department of Energy. You must contact the persons listed in the credit lines before using those images.
Creative Commons Zero (CC0) Public Domain Designation images
Images with a CC0 licence mean that you can: copy, modify, distribute and perform the work, even for commercial purposes, all without asking for permission. There is no legal requirement that you attribute the image, only an expectation that you will voluntarily do so if requested.
The Art Institute of Chicago - CC0 on the lower left of the artwork. You can filter searches to Public Domain: https://www.artic.edu/collection?is_public_domain=1
The museum requests that you include the following caption with reproductions of the images: Artist. Title, Date. The Art Institute of Chicago. This information is available on the object page for each work.
The New York Met Fifth Avenue - Open Access icon at the lower left of the artwork. You can filter searches to Open Access: https://metmuseum.org/art/collection/search#!?showOnly=openAccess&offset=0&pageSize=0&perPage=20&sortBy=relevance&sortOrder=asc&searchField=All
OA does not require any attribution or credit, but as an educational institution, the Museum hopes that you will include a citation. All the information necessary for proper citation of a CC0 image is available on the Collection page for that object.
A sample object credit line format is as follows: "The Metropolitan Museum of Art, New York," followed by the specific information identified as the "Credit Line" on the Collection page for each work of art. The citation could also include the URL www.metmuseum.org.
Pixabay https://pixabay.com/ A collection of over 1.6 million royalty free stock photos and videos all published under a CC0 licence.
Public Health Image Library (PHIL) https://phil.cdc.gov/
Most images are in the public domain and free to use. Note some images are copyright protected, so check directly beneath the image to see the fair use statement. Permission is not required to use the public domain images but they ask that you credit the original institution and contributor, when known, whenever the image is used in any publicly distributed media (see: https://phil.cdc.gov/FAQ.aspx#question7)
Unsplash https://unsplash.com/ Half a million stock photographs published under a CC0 licence