Each site and, in some cases, each image has its own "terms of use." Be sure to review Terms of Use for all images.
​Usage of some images for non-profit research or study might be allowed under Fair Use but you must cite your sources to give credit to the creator.
Attribution for used images for a published research paper or book is likely different than for a class project or presentation where a citation to the original source is sufficient.
David Ramsay Hay's mapping of colour onto musical notes, a diagram from his The Laws of Harmonious Colouring (1838) via the Public Domain Review.