Be sure to check image licenses for information about how to credit images you find from free stock image providers. A great blog post with a variety of free stock image sites can be found here: http://bootstrapbay.com/blog/free-stock-photos/.
Stock images are pictures that designers use to illustrate publications. Generally, stock image providers require a per-image payment or a subscription, although some are free. Cornell University Library does not subscribe to stock photo services. But in the column opposite, you'll find examples of free and fee-based stock image sources online.