Those people who love old photos and history are now indebted to an academic who's been busy sharing his passion, and his knowledge of coding, to publish millions of copyright-free images on the photo sharing site Flickr.

Kalev Leetaru has already uploaded 2.6 million pictures to Flickr, which are searchable thanks to tags that have been automatically added.

The images images are from books published over the last 500 years, and have been difficult to access until now.

Leetaru, who is a fellow at Georgetown University in Washington, wrote a program that is helping take photos from public domain books in the Internet Archive. He has extracted over 14 million images from 2 million eBooks that span over 500 years of content. Importantly, because the books have been processed with optical character recognition, he has been able to attach about 500 words before and after each image. This means you can now search, view, and read about each image in the collection. Think full-text search of images!

So, this new Flickr archive accomplishes something quite important. While other projects (e.g., Google Books) have digitized books and focused on text — on printed words – this project concentrates on images. Leetaru told the BBC:

"For all these years all the libraries have been digitising their books, but they have been putting them up as PDFs or text searchable work. They have been focusing on the books as a collection of words.

"This inverts that. Stretching half a millennia, it's amazing to see the total range of images and how the portrayals of things have changed over time. Most of the images that are in the books are not in any of the art galleries of the world - the original copies have long ago been lost."

Flickr users can search the growing catalog and use just one single search term to access photos as each image page offers text. In case you’re wondering, all images can be downloaded for free. They’re all public domain.