[sarcasm]Ooh yes, deny from all, that won't do anything bad![/sarcasm]
I suppose you're talking about directory listing? When you go to a directory that doesn't contain an index file, you get a list of all files in the directory?
The solution is easy, put a .htaccess file in the directory (or any parent directory of that directory) containing the following (or append it to the existing .htaccess document).
Note: deny from all will block ALL access to all files in that directory. Including the access required to view images when you're allowed to.