Most countries have their own instance of Google Image Search.  Identify the domain for the country.   A list of all domains can be found at: Domain Name Registries Around the World.

A list of sources for public domain and relatively unrestricted images.

• Yale University
  • Open Access Policy
  • Yale Center for British Art Collections Search
  • Yale University Art Gallery Collections Search
• National Gallery of Art
  • Open Access Policy
  • Collections Search
• The Getty
  • Open Content Program
  • Getty Search Gateway
• LACMA
  • Terms of Use
  • Collections Search
• Walters Art Museum
  • Collections Search
• Rijksstudio
  A service of the Rijksmuseum, Rijksstudio offers high resolution images (4500x4500px usually) for public domain works via a direct download on the site.
  • For ordering TIFFs: https://www.rijksmuseum.nl/en/photoservice
  • Rijksstudio search and browse: https://www.rijksmuseum.nl/en/rijksstudio?ii=0&p=0&from=2016-08-02T20%3A37%3A02.4603358Z
• NYPL Digital Collections
  You can limit your search to public domain images only.
  • Explanation of public domain collections: https://www.nypl.org/research/collections/digital-collections/public-domain
  • Public domain search results page: http://digitalcollections.nypl.org/search/index?utf8=%E2%9C%93&keywords=&filters%5Brights%5D=pd
• Library of Congress Digital Collections
  This collection contains a mix of restricted and unrestricted works. However, each subcollection has detailed information regarding usage restrictions. Some material does not have restrictions (for example, drawings from the Lamb Studios Drawings)
  • Master list of rights for subcollections: http://www.loc.gov/rr/print/res/rights.html
  • Collection search: https://www.loc.gov/collections
• Wellcome Images
  Historical images are available for free, high-res download and are licensed under CC-BY 4.0 (creative commons attribution-only license). The images must be credited.
  • Policy: https://wellcomeimages.org/ (see “Services” section)
  • Collections search: https://wellcomeimages.org/
• Europeana Art History Collection
  Europeana allows you to limit to “Free re-use” images on the search results page.
  • Terms and policies: http://www.europeana.eu/portal/en/rights.html
  • Search for art history images: http://www.europeana.eu/portal/en/collections/art-history?q
• Flickr Commons
  This is a collection of material from various cultural heritage institutions, which contribute images of works believed to be in the public domain.
  • Flickr Commons rights statement explanation: https://www.flickr.com/commons/usage/
  • Commons search page: https://www.flickr.com/commons
• Flickr
  The main Flickr website also allows you to filter by license. Many images are licensed using creative commons licenses, some of which allow for modifications (like cropping) and commercial use. Appropriate filters would be: Commercial use & mods allowed, No known copyright restrictions, U.S. Government works
  • Flickr search: https://www.flickr.com/search/?text=&license=4%2C5%2C9%2C10
• OpenGLAM
  OpenGLAM is a directory of open access image collections online.
  • OpenGLAM directory listing: http://open-collections.okfn.org/items/browse?search=&type=18
• Public Domain Review
  Public Domain Review is a curated online publication that also links to high-resolution images. The images page allows you to filter by rights. Some of the images come from Wikimedia Commons.
  • Public Domain Review Rights labeling explanation: http://publicdomainreview.org/rights-labelling-on-our-site/
  • Public Domain Review Images page: http://publicdomainreview.org/collections/?medium=image