You want images only be available to users with a Wikipedia login? This would mean by far most people would no longer see images in Wikipedia articles.
As far as I am aware there is no such thing as a silver bullet anywhere when it comes to security.
Its like moving your SSH port from port 22 to some other random one. Will it stop advanced scripts from scanning your server and finding it? No, but it sure as hell will cut down the noise of unsophisticated connections which means you can focus on the more tough ones.