http://www.javascriptkit.com/howto/htaccess3.shtml
Sometimes you need to password protect a directory and your only available option is using .htaccess. This comprehensive guide will show you how to do that as well as using .htaccess for other things such as preventing hot linking and blocking bots.
*Note:
You can also prevent image hot linking with PHP and mod_rewrite.
http://alistapart.com/articles/hotlinking/