OER stands for Open Educational Resources.
When you use images that you did not create in your work, it is important to make sure that you don’t violate anyone's copyright. The images you find on the Web may not be available for reuse at all or only available for reuse if you pay license fees or ask for permission.
There are resources that provide quality images that you can use freely as long as you attribute the images to their creators and meet the terms of the license.
How to search for a freely available image in Flickr
"Rays of Hope" by Mick Talbot is licensed under CC BY-NC-SA 2.0
How to search for a freely available image in Google Images
"The last hope" by Johnny Martínez is licensed under CC BY-ND 3.0
How to search for a freely available image with Creative Commons
1. Go to https://ccsearch.creativecommons.org/ and type your keywords in the search box. This searches hundreds of public repositories, including the Metropolitan Museum of Art and New York Public Library. You can limit your search to specific repositories if you wish.
2. Click on the image to see the attribution information, including a link to the source and the license type.
3. Click on the license type to see information about that license and attribution requirements.
How to attribute images to their creators
There is a tool to help you create attributions.