Use pxhere.com to find images and stock photos free of copyrights under a CC0 license.
CC Search is a tool that allows you to browse images, audio and video that can be reused.
NOA is a search engine for scientific images from open access publications. It is based on automatic harvesting and indexing of figures from the fields of engineering and technology. NOA is using the infrastructure of Wikimedia Commons and Wikidata.
The British Library offers access to more than a million images from its digitised collection of books from the 17th, 18th and 19th centuries. Everyone is encouraged to use and re-use the public domain images: