There you can search broadly via a federated platform for images with CC licenses from many collections. These results you can be assured are usable, remixable under these licenses. Plus it provides a one click attribution creator. I’d say it’s the best search tool to recommend for a wide audience of OER creators.
Soon it integrated into the WordPress platform (currently WordPress.com sites and self hosted ones using the JetPack plugin) as a means to find and use images when composing in WordPress.
Related, but as another WordPress resource, they have created and are expanding a collection of CC0 licensed photos aimed at again integrating as a tool to use in their platform, but also as an open resource. Have you seen the WordPress Photo Directory?
If these are new to you or you are looking for some materials to reuse/use to teach others how to use these sites, WordPress has also published lesson plans with examples
And you can see they are on board as these materials are shared under aCreative Commons Attribution-ShareAlike 4.0 International License.