Well I have missed something important... the fact that it should be...
public class ImageController : ApiController{}
However I have also found that the breeze web API config does conflict with the normal web API config so for the time being I've had to change BreezeWebApiConfig.cs to the following
public static class BreezeWebApiConfig {
public static void RegisterBreezePreStart() {
GlobalConfiguration.Configuration.Routes.MapHttpRoute(
name: "BreezeApi",
routeTemplate: "api/{controller}/{action}"
);
}
}