I’m just going to quote from this new BBC article, Millions of historic images posted to Flickr:
An American academic is creating a searchable database of 12 million historic copyright-free images.
Kalev Leetaru has already uploaded 2.6 million pictures to Flickr, which are searchable thanks to tags that have been automatically added.
The photos and drawings are sourced from more than 600 million library book pages scanned in by the Internet Archive organisation.
I’m adding this post to The Best Online Sources For Images.