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.
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)
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.