River scene, the Netherlands - Glass negative photogrpah. Public domain.

This collection as well as all massive collections on Picryl.com required two steps: First, we picked a manually curated set to train AI vision to recognize windmills, and after that, we run image recognition through all 25M+ images in our database. All media in the collection is in the public domain. There is no limitation on the dataset usage - educational, scientific, or commercial.