This is an excellent round up of 53 free image sources posted in May 2014 on buffersocial by Courtney Seiter Most of these sights are sampler sites for professional (for pay) images sites such as iStock so, as always,k be careful where your clicks lead you.
https://blog.bufferapp.com/free-image-sources-list
One of the presentations I had during class was 10 Tips for making Beautiful Slideshow Presentations by Edahn Small on slideshare.
You can view the entire presentation from the link, but here are the images sites he recommended, updated to November of 2014. These sites mix free and for sale images, so read the licenses and the explanations with care.
http://www.everystockphoto.com
I make my photos available on flickr.com which is part of yahoo. We used the creative commons option at the bottom of the advanced search page to search for images that can be used in presentations.
Some of the sites above include flickr in their searches as well.
Google has made its image search more useful by adding a License filter to its search tools.
After entering search terms and choosing Images, click on the Search Tools button to allow filtering by Size, color, type of image, and license. The clip above has Photo for image and Reuse with mofication selected for type of image and license.