Publish a search engine sitemap of all media items in the Umbraco media library.
The sitemap can be accessed at https://hostname/sitemap/mediafiles/.
The media URLs are relative, and are appended to the value of a cdnDomain app setting in web.config, so to achieve a URL of https://www.example.org/media/1234/example.jpg you need the following in web.config:
<configuration>
<appSettings>
<add key="cdnDomain" value="https://www.example.org" />
</appSettings>
</configuration>