There are countless online image sites with more and more being added everyday. By far, the most commonly used site is Google Image. To learn about using Google Image, go to the Google Image Page.
While Google Image pulls from a huge amount of sites and is easy to use, it is far from perfect. First, it relies on whomever made the original websites to accurately describe and tag the photos. You can get some really interesting things popping up, even with filters on. Second, you have no idea of the usage rights without going directly to the website and even then, you may not know. It is up to you to follow copyright laws. For more information on copyright, see the copyright section of this guide. And anytime you are in doubt, contact the site owner.