Repositories and search strategies
Searching for OER is not easy. Because there is not one overarching repository, but an immense number of possibilities.
In any case, the search engine of Creative Commons is recommended. Here you can find CC-licensed images gathered from a variety of sources. If you decide to use a particular image, you will be shown the required licence information.
In addition, two search strategies can be helpful:
The first search strategy is to filter the content in search engines or on platforms according to licence. This filtering option is sometimes somewhat hidden. On Google, for example, the desired licences can be selected in the advanced search settings. Filter options can also be found on media platforms such as Youtube or Flickr.
The second search strategy is based on repositories or specific OER platforms. For the German-speaking countries, for example, the search engine ‘Wir Lernen online‘ (= We are learning online) is currently being developed. For the cultural sector, you can find open contents at the Open GLAM network. GLAM is the abbreviation for galleries, libraries, archives and museums. Many of these institutions make their exhibits openly available.
Most likely, in your country there are other OER repositories or repositories where content can be found under a CC licence. Feel free to add them in reply to this post.