We're purchasing access to a couple databases, which makes it much easier to know that they are authentic, and that you can legally use the images for educational purposes.
Finding Images on the Web
Images are readily accessible with Google, but it is important to determine both:
1. Is it authentic, the real thing, or is it fake or been altered?
Pay close attention to the site you're getting it from. Is it a museum, or someone's blog? Be picky!
Looking for historical, everyday photographs? Try limiting to "historical society" to pull results from [whatever place] historical society websites.
2. Can I legally use it?
Is the image in the public domain or copyrighted? If the image is copyrighted, permission from the artist or owner is required. Here are a couple free sites.