The following are a variety of resources and examples of what you can find.
|Google Image in the public domain.|
Getty Images. They may have usable images for blogs and sharing on social media. Enter a search term, on the search results page, click an image. On the details page you will see information on embedding or a social media icon if the image can be shared.
|Jean Henri Marlet Wellcome Library, London|
Wellcome Images. "Thousands of images and centuries of medicine, science, society and culture." Some "images are available to download from this site free of charge, for use both commercially and non-commercially under a Creative Commons Attribution Only – CC-BY 4.0 licence."
Flickr Commons. Shares the "world's public photography archives." Search for images with "Any License" option set to "No known copyright restrictions."
|Smithsonian photograph: Postcard of the Delaware and Hudson train wreck|
Or look for some of the following institutions on Flickr and use the search bar under their name.
|U.S. National Archives’ 1886-model bicycle for two.|
The British Library.
National Library of Ireland.
Internet Archive Book images.
The New York Public Library.
U.S. National Archives.
You might also like my Pinterest Board with more examples.