According to the documentation, putting them in a folder other than WEB-INF
is all you have to do.
By default, App Engine makes all files in the WAR available as static files except JSPs and files in WEB-INF/. Any request for a URL whose path matches a static file serves the file directly to the browser—even if the path also matches a servlet or filter mapping.
A request to /img/famousActors/willSmith.jpeg
should work without additional configuration.