These online version of texts are especially useful for searching throughout a work for a particular word or phrase.
Quick Links to databases used:
Major search engines link Google and Bing have image searches as well. Other major sources of images include Flickr and Wikipedia. Use these sites to effectively search these sites, and limit to images that are free to reuse.
Most image sources will include information about the copyright status. Many sources will allow you to reproduce images for teaching or other non-commercial uses, though for publication you must obtain permission from the copyright holder. Always remember to attribute the source of the image when incorporating it into your own presentations.
Some image search engines or websites allow you to limit your search to images licensed with Creative Commons licenses that may allow re-use. Often this is on the 'Advanced Search' screen. See the specific license to see if you can modify the image (make derivative works), use it for commercial purposes, or need to make attribution to the creator. When you use a Creative Commons image, you will need to link to the terms of the license on the CC website.