National Gallery of Art Open Access

The Washington gallery freed its images early, launching NGA Images in 2012, and it now offers more than 50,000 high-resolution pictures of public domain works under CC0, free to download and reuse without asking permission. That makes it a dependable source when a lesson needs a crisp Vermeer, Turner or Leonardo's Ginevra de' Benci. It also shares its collection data openly and has placed tens of thousands of images on Wikimedia Commons, so the same pictures turn up in Wikipedia articles students already use. Interpretive text is lighter than at the Met, so pair it with a good essay when context matters.