online image libraries (CC or public domain content)

• Openverse— An extensive library of free stock photos, images, and audio, available for free use. (All Openverse content is under a Creative Commons license or is in the public domain.)

museums (diverse content, but especially great at historical images, artifacts, etc)

• Wellcome Collection — Highly visual, highly interesting. “They contain an array of topics relating to health and the human experience in many forms, from recently published books to historical manuscripts and objects.” Free to use. Everything I’ve looked at so far has been licenced under the Public Domain Mark.

• Cleveland Museum of Art — lots of antiquities etc under the Public Domain Mark. Copyright is either way very clearly labelled, and downloading is made easy.

nature

• Saxifraga Gallery: Free Nature Images — Free stock photos of animals, plants and landscapes. All images are available on the website at a resolution of 1200 pixels. Bias towards European results. English searches work well, but I have noticed it’s often also worth trying in German. Plants seem searched by their searched by their scientific/Latin names.