Here is a collection of links to various resources around the net that I have found handy. Note that they may have changed their license terms or other bits since I added them to the list so make sure to read through any agreements etc before using them.
Images
Comic Book Plus – Everything from newspaper serials to full comic books that are in the public domain.
Old Book Illustrations – public domain images scanned from old books. Offered up royalty free. The image at the top of this page (An ocean of books) is from OBI.
Unsplash – Free stock photographs released under their Unsplash License.
Charts, diagrams and similar
Data Wrapper – create high quality maps, tables and charts. Has a free option.
Diagrams.net – from timelines to org charts, system layouts or illustrative pictures, Diagrams.net (formerly Draw.io) is an invaluable resource.