The mother of them all, Google Images casts a wide & broad net for you to look for any and all images you might be looking for.
Flickr pulls from the uploads of its users, which number in the millions. A fantastic thing about Flickr, unlike Google Images, is that you can search for images that reside in the Creative Commons**.
You can not only search Flickr for CC images, you can search the CC folks directly!
**What is the Creative Commons, you might ask?
Creative Commons is a platform that allows users to simultaneously protect their creative works while also sharing them with the world,. in ways that they personally deem suitable and acceptable.
Or, the way that the CC people put it themselves:
"The Creative Commons copyright licenses and tools forge a balance inside the traditional “all rights reserved” setting that copyright law creates. Our tools give everyone from individual creators to large companies and institutions a simple, standardized way to grant copyright permissions to their creative work. The combination of our tools and our users is a vast and growing digital commons, a pool of content that can be copied, distributed, edited, remixed, and built upon, all within the boundaries of copyright law."
- Creative Commons Licensing Considerations.