Images without copyright or with a license that allows re-use (e.g. Creative Commons) can always be used for any publication.
Databases with a lot of open content, by searching or filtering on specific licenses:
To search all these databases and more in one go, use the search function for Open Content on the Creative Commons website.
For more information, check our page on Finding open content!
See also our page Photographs & audiovisual work on our Copyright Information Point.