Sourcing public domain artworks that are freely available online can be a lifesaver for set dressing but is another path that can take you down a rabbit hole. Here's a set of archives that have thoroughly researched data and clearly flagged signposts copyright status to help you.
The main question I hear is - "If it's a wood block from the 13th century it must be fine to use right? Which is logically correct but things can sometimes be a little more complicated than that. Sometimes
There are additional protections on that digital file which was made by someone and will often have revenue attached
Someone may have taken the original public domain print and then changed it before publishing again online - which mean they might own the copyright on the changed image.
You can bypass some of these unknowns by heading straight to a reputable source. The sites below have all had their terms reviewed and are generally agreed to be pretty favourable with some lawyers but you will always check with the lawyers on your production first. As always - these resources aren't bulletproof - so anything you want to use should always go to your clearance person or lawyer to double check
Gothic Art
Renaissance
Dutch Golden Age
Baroque
Romanticism
Realism
Impressionism and Post-Impressionism
Asian Art
Mesopotamian, Egyptian, and Greco-Roman Art
Ancient American Art: Mesoamerican and South American cultures. Indigenous and Pre-Columbian Art
African Art
Chinese, Japanese, and Korean Art: Buddhist and Hindu art reflecting religious traditions.
Late Medieval and Early Renaissance Art
Baroque and Rococo
Neoclassicism, Romanticism, and Realism
American Art - Colonial and Federal period portraits, and early 20th-century Modernism
Impressionism and Post-Impressionism
Abstract Expressionism, Pop Art, Minimalism, and Postmodern works.
Islamic Art
Decorative Arts and Design
Mesopotamian, Egyptian, and Greco-Roman Art: Objects from Olmec, Maya, Aztec, and other cultures of the Americas.
African Art
Chinese Art: Calligraphy, ink paintings, ceramics, bronzes, and jade.
Japanese Art: Samurai armor, ukiyo-e prints, ceramics, and screens.
Indian and Southeast Asian Art: Hindu and Buddhist sculptures, miniature paintings, and textiles.
Islamic Art: Intricate calligraphy, ceramics, carpets, and metalwork.
Late Medieval and Early Renaissance Art
Renaissance Art: Paintings, sculptures, and decorative arts from the 14th to 17th centuries.
Baroque and Rococo: Dramatic and ornate works emphasizing movement and detail.
Romanticism, Neoclassicism, and Realism: Art reflecting 18th- and 19th-century European themes.
American Art
Impressionism and Post-Impressionism
Modern and Contemporary Art
Prints and Drawings -
European and American Prints,
Japanese Ukiyo-e,
Political Cartoons and Caricatures:
Calling cards, postcards, and printed materials with artistic designs
Advertising and propaganda art from the 20th century.
Photography
Maps and Atlases
Modern and Contemporary Art
Children’s Book Illustration
Architectural Drawings and Design
Historical and modern architectural plans
Ancient Art, Pre-Columbian and Mesoamerican Art and Native American Art
African Art
Traditional masks, sculptures, and ceremonial objects from sub-Saharan Africa.
Contemporary African works showcasing modern interpretations of traditional themes.
Asian Art, Chinese Art, Japanese Art, Indian and Southeast Asian Art: Hindu and Buddhist sculptures, Korean Art: Celadon ceramics,
Late Medieval and Early Renaissance Art
Renaissance Art:
Baroque and Rococo:
Neoclassicism and Romanticism:
Impressionism and Post-Impressionism:
Modern and Contemporary Art
Ancient Art, Greek and Roman Art, Egyptian Art, Etruscan and Mesopotamian Art:
Late Medieval and Early Renaissance Art
Renaissance Art
Baroque and Rococo Art
Neoclassical
Romanticism and Realism,
Impressionism and Post-Impressionism
Modern and Contemporary Art
Photography
Decorative Arts and Design
Prints and Drawings
Photography
Posters and Graphic Design
Fine Art Prints
Maps and Cartographic Art
Manuscripts and Illustrated Texts
Performing Arts and Music
Decorative Arts
Folk and Traditional Art
Comics and Cartoons
Late Medieval and Early Renaissance Art
High Renaissance
Mannerism
Baroque Art
Rococo
Neoclassicism and Romanticism
Impressionism and Post-Impressionism
Realism and Pre-Raphaelitism