This override is used by OpenNext and more specifically by the image server to be able to load images from a custom source. Images are not bundled with the image server and are loaded from the source specified in the override.

This is used for internal image only (i.e. src without host). External source are already handled by the image server.

If you want to better understand how to implement your own ImageLoader, the easiest way would be to take a look at the existing included ImageLoader (opens in a new tab).