One of the key concerns when seeking images online is image quality. A wealth of images may be found online; however, many of those images are of poor quality, or low resolution, and become pixelated when projected or printed. Copyright is another concern when pulling images from the web. While the fair use sections of the United States copyright acts generally allow for use in an educational setting, it is to your benefit to pull images from the web with caution, the same as you would when scanning pictures from a text.
The Guide to Finding Images Online offers links to image collections around the world, many available for educational use, and most containing high quality, higher resolution images. There are a number of sites to seek images from; consider this list a starting point.