Creative Commons Search is a tool that searches several sources (Flickr, Rijksmuseum, Metropolitan Museum of Art, etc.) for images, music, video, and other media protected under a Creative Commons Licence.
Wikimedia Commons contains over 27 million freely usable media files, including images, sounds, and videos. The highest quality items are ‘Featured’ by the site and appear in picture of the day and picture of the year selections.
Flickr Commons is a repository for historical images from cultural heritage institutions such as libraries, museums, and archives. Most material has “no known copyright restrictions” and is free for noncommercial use. Please check licences.
Pixabay is a collection of over 1 million high-quality stock-like images. All images, except the images watermarked with Shutterstock can be used with no restrictions. Users can 'like' or favorite images, providing a means of evaluation.
Unsplash is a collection of high resolution photographs which are licensed by Unsplash so you can copy, modify, distribute and use the photos for free, including commercial purposes, without asking permission from or providing attribution to the photographer or Unsplash.
All images require attribution, and are licenced under various levels of Creative Commons licensing unless licensing is not listed on an image which means you should contact the journal. Collection is from The Open Access Subset of PubMed Central (PMC), a free full-text archive of biomedical and life sciences journal literature at the U.S. National Library of Medicine, the Indiana University hospital network, the Orthopaedic Surgical Anatomy Teaching, and the Collection at the USC Digital Library.