Using public domain art in commercial work
A painting in the public domain can go on a print, a mug or a book cover without anyone's permission. On Artnap every one of the 466,483 works has a license line on its page, so you can see the status before you start. Works marked public domain or CC0 can be used for any purpose, commercial products included, and no payment is due. With CC0 the person behind a work has "dedicated the work to the public domain by waiving all of his or her rights to the work worldwide". Source: Creative Commons.
Works marked CC BY can be sold on too, as long as the credit shown on the page goes with them. That is the only extra step. The Rijksmuseum explains CC BY in its own data policy. "This allows you to use the material, as long as you acknowledge the Rijksmuseum as the source." Source: Rijksmuseum data policy.
Why the country matters
Copyright lasts for different lengths of time around the world. A work can be public domain in the United States and still protected somewhere else. Each Artnap page says where the work is in the public domain. If you sell in another country, check the rule there as well.
This page is general guidance and not legal advice. For a large print run or a licensing deal, a short talk with a lawyer is worth the cost.
What the museum record adds
Every artwork page names the museum that holds the work and links to its record. The record shows where the image came from. If a shop or a client asks about the source of a design, a museum link answers that quickly.
Museums such as the Rijksmuseum, the Art Institute of Chicago and the National Gallery of Art release their public domain images under CC0. That is the clearest status for commercial use. Museums often ask for a short credit too. The Art Institute of Chicago "requests that you include the following caption with reproductions of the images". The caption it gives names the artist and the work.
File size for print
The standard copy on Artnap is a 2,500 pixel JPEG with no watermark. It prints well at about 21 centimeters at 300 dpi on the long side. For posters and canvas prints Artnap Pro adds the museum's master file, which is often several times larger.
Things to avoid
A public domain painting has no copyright left, but a museum's name and logo are not. Do not suggest that a museum made or endorsed your product. A photo of a sculpture or another object can carry rights of its own. Read the license line of that work before you use it.