Where to find large public domain painting files
Artnap brings the open access releases of 72 museums and libraries into one search. It holds 466,483 public domain works and 124,073 of them are paintings. Every work has a 2,500 pixel JPEG with no watermark and no account needed. Each page links to the museum's own record, so you can check the source and the license before you use the image.
If you need a bigger file, the best place is usually the museum that holds the painting. Most large museums now publish their public domain collections on their own sites, and many of them release the images under CC0. Artnap Pro adds the museum's master file for each work, which is often several times larger than the standard copy.
The museums with the largest open collections on Artnap
The Rijksmuseum in Amsterdam is the biggest source by far. Artnap holds 134,086 of its works. Of these 6,293 are paintings. Its online collection offers the full files as well. The museum's data policy says that for material marked this way "the Rijksmuseum has waived its rights." Source: Rijksmuseum data policy.
The Metropolitan Museum of Art in New York comes next with 33,828 works on Artnap. The Met marks its public domain images as Open Access, and anyone can save them from the collection pages.
The National Gallery of Art in Washington has 25,727 works on Artnap. The Art Institute of Chicago follows with 24,190. Both museums publish their public domain images under CC0. Chicago's open access images are works "believed to be in the public domain or to which the museum otherwise waives any copyright it might have." Source: Art Institute of Chicago.
Smaller collections are worth a look too. The Finnish National Gallery has 2,759 paintings on Artnap. The Staatliche Kunsthalle Karlsruhe has 2,162.
Searching across museums at once
Each museum site only shows its own collection. On Artnap a search for a subject, a color or an artist returns works from every museum in one list. You can then open the museum record from the artwork page.
The full list of museums and the number of works from each is on the Museums page at artnap.com/museums.
How to check that a painting is public domain
Look at the license line on the artwork page. Works marked public domain or CC0 can be used for any purpose. Copyright rules differ from country to country, and the page says where the work is in the public domain. Check that against the rules where you live before you print or sell anything.
A ranked list of the fifteen largest collections, with each museum's open access policy, is in The largest open access museum and library collections on Artnap.