Abstract Fish Patterns by Lynn Greyling, Public Domain
The webpages below are listed in the order of least restrictive to more restrictive. However, even the "most restrictive" webpages listed below often have less restrictions than a even simple Google image search.
"Creative Commons is a nonprofit organization that enables the sharing and use of creativity and knowledge through free legal tools" (Creative Commons website). By using images from the Creative Commons, it allows you to use images for free with some restrictions such as giving credit (attribution) to the original creator.
For more information on Creative Commons, go here.