Google Advanced Image Search (Scroll all the way down to the bottom to "Usage Rights") 
Usage rights correspond to Creative Commons Licenses (see the abbreviations above in red) which allow free use and sharing, but not always modification or commercial use.
Other Ways to Locate Openly-Licensed Images
Creative Commons (Cultural Heritage) Search (beta) (Images only with an emphasis on openly licensed cultural heritage images)
CreativeCommons Search Search for openly licensed images in: Europeana, Flickr, Pixabay (public domain), Open Clip Art Library, and Wikimedia Commons. (Also includes options to find music & video)
Wikimedia Commons (Terms: CC and public domain)
Creative Commons Image Directories (Terms: CC and public domain)
The Noun Project (search CC-licensed and public domain icons by keyword)
Figshare (Terms: CC-BY Licensed) research-related images, data etc.
Compfight (Terms: CC and Commercial)
Pixabay (Terms: CC0 = public domain)
PhotosForClass (Creative Commons licensed images from Flickr -- required citation information is automagically added!!)
Various public domain image websites (Terms: public domain, but read the fine print)
These are just a few of the many places to find openly-licensed images or illustrations.
Reverse image search: TinEye and Google Images Reverse Search
* Public Domain: Works whose exclusive intellectual property rights have expired, have been waived, forfeited, or are inapplicable.
* Find more information on the Public Domain
Don't forget to attribute CC-licensed images ---------------->
Attribution is ALWAYS REQUIRED when using works that have a Creative Commons license. Learn how to attribute CC-licensed works by following these best practices -- or use a browser plug in such as OpenAttribute to help you.
The most common forms of attribution include the three elements, two of which are hyperlinked:
(c) Author Name "Name of work [hyperlinked to where you found the work]" License [hyperlinked to license]
For example:
SAMPLE ATTRIBUTION: (c) Quinn Dombrowski "Red and blue" CC BY-SA 2.0
License terms on some sites (this one is from Flickr.com) may appear as Creative Commons icons or appear when a hyperlink is moused over.
Learn how to attribute CC-licensed works by following these best practices -- or use a browser plug in such as OpenAttribute to help you.