These four sources will allow you to search for images that are labeled as available for use. Some of them have no restrictions; some only permit academic use; some permit use only if you credit the source; some permit you to make substantial changes and some do not. Look carefully at each image that you find for license terms before using it to be sure it is legally available for your purposes.
1. Go to 
2. Search for your desired image by keyword
3. If you need a specific type of license permission - one that allows reuse for commercial purposes, one that allows reuse for noncommercial purposes, etc. - limit your search by License Type
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Attribution Allows any reuse with credit to the original creator, even commercial uses Attribution noncommercial Allows reuse for noncommercial purposes with credit to the original creator; any changes must be described in the credits Attribution noncommercial no derivatives Allows reuse for noncommercial purposes with credit to the original creator but you may not make changes of any sort Attribution noncommercial share-alike Allows reuse for noncommercial purposes with credit to the original creator; any changes must be described in the credits; you must use the same license when you redistribute the changed image |
4. Click on the License link under the image on the image page to find the exact terms of the license. There are different varieties of each type.
5. If there is no good link to the specific license for an image, visit its hosting site.
Go to 
Enter your search term, and click Search.
On the results screen, click Tools below the search box to open an array of tools for narrowing the results.
Select Labeled for Reuse or other appropriate usage category from the Usage Rights drop-down menu.
The results visible will narrow to those that fit your usage rights choice
Browse to choose a desirable image, and check EACH IMAGE for a specific license statement
Link back to and cite the image source AND license statement as you reuse the image in a presentation or slide
Go to openverse to search a wide range of open images, music, sound effects, and video.
You must read the license for EACH ITEM you link to or re-publish.
You must ALWAYS cite the source and link back to the specific license.
Go to the explore page at 
Search for your desired image topic
Click on the Any license drop-down menu from the link at the top left of the search results.
Select the desired permission level to see images
Check EACH IMAGE for a license statement to make sure that it allows your desired use. Some on Flickr do not allow re-use at all.
Link back to and cite the image source AND license statement