Different rules apply if you are using an image as part of a class assignment versus creating something you plan to publish.

Google Searches:
Images on a general Google search are likely copyright protected, but there is a way to narrow down the results to usable pictures and graphics.
Except where otherwise noted, content on this site is licensed under a Creative Commons Attribution 4.0 International license.
All pictures are public domain. Keyword search and then limit to images.
All images are public domain. Please credit: "NASA/JPL-Caltech" or "Courtesy of NASA/JPL."
Free clip art.
"Open access" availability does not mean that the images in these collections are in the public domain--rather, it refers to their accessibility for research and educational purposes. For copyright and use restrictions, locate and read the image use policy for each collection.
NGA Images is a repository of digital images of the collections of the National Gallery of Art. On this website you can search, browse, share, and download images. A standards-based reproduction guide and a help section provide advice for both novices and experts. More than 51,000 open access digital images up to 4000 pixels each are available free of charge for download and use. NGA Images is designed to facilitate learning, enrichment, enjoyment, and exploration.
Over 73,000 images organized into nearly 900 topical portfolios. Portfolios are grouped by subject. The entire database is searchable. Incorporates AICT - Art Images for Teaching.
Art Images for College Teaching (AICT) began as a personal project dedicated to the principle of free exchange of image resources for and among members of the educational community. While the AICT site is maintained and distributed under the general auspices of the Minneapolis College of Art & Design (MCAD), this institution is not responsible for content or use thereof. All work on the AICT project has been voluntary, and MCAD-affiliated personnel have contributed innumerable hours of research, clerical, and design effort on a pro-bono basis. Use of the images displayed on this website has also been contributed on a non-royalty basis for the public good.
AICT certifies that the image copies provided hereunder, the originals being the copyrighted intellectual property of art historian and photographer Allan T. Kohl (unless otherwise credited), are licensed for non-profit use as set forth below, and that the art and architectural works documented are, to the best of its knowledge, within the public domain
Nearly one million digital images taken from the Library's 50+ collections of prints and photographs. Included are images from the the National Child Labor Committee Collection, the U.S. News and World Report Magazine Photograph Collection, the Library of Congress Baseball Card Collection, and the Collection of Architecture, Design and Engineering Drawings.