Disallow access to member files when they are in public file system
To Apache or any other web server, seeing 

&lt;img src="http://www.foo.com/bar.jpg"&gt;


in html and a user typing http://www.foo.com/bar.jpg into their browser directly is basically the same thing so the quick answer is "no, not really, at least from a drupal standpoint."  You could obfuscate the path with some javascript trickery but that's about it.

Now, you could look into Apache referrer access, a quick link about allowing access from specific referrers might shed some possibilities on it for you, but now you are certainly outside of drupal :)