A nonprofit resource that provides over 1.6 million digital images in the arts, architecture, humanities, and sciences with an accessible suite of software tools for teaching and research.
Many Flickr users have chosen to offer their work under a Creative Commons license, and you can browse or search through content under each type of license.
A wide array of images from across the disciplines, but quality can be uneven and may not include attribution of original source. If you plan to post images found via Google on the Web or use them in a publication, ensure you have permissions from the original source.
Public Access Image Databases
A list of sources for public domain and relatively unrestricted images.
Rijksstudio A service of the Rijksmuseum, Rijksstudio offers high resolution images (4500x4500px usually) for public domain works via a direct download on the site.
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)
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.
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 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
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.