If you wish to reuse an image or diagram, reference it. If you do not know the original source to cite, you can use image reverse search engines such as Tineye.
The site will be able to upload the image or insert a URL. The filters will enable you to find the other places where the image has been used; following the links, you will see either the source or information about it.
The author takes no responsibility for the content of this page, which is not intended as binding legal advice. The views are about copyright in the UK Higher Education sector context.