Incredibleart.org offers a wide variety of free clip art illustrations. The images are available to copy directly from the page or download to a computer. Online search engines provide additional resource options.
Stockvault.com is another website that provides stock illustrations for free. The images on both sites can be searched by image descriptions, such as flowers or clouds. Background images such as seasons or landscapes are also popular. As of June 2015, there doesn't appear to be any limitations on the use of the images. Once downloaded, the image can be altered from color to black and white or made smaller or larger to taste.