wilderness - 5:34 pm on Feb 3, 2013 (gmt 0)
It's NOT your htaccess that applies the script, rather your PHP.
Applying watermarks in this manner(rather than embedding watermarks) presents to users some very similar to to the Java-no-save-image option.
The false-prevention is easily circumvented.
In any event, if you prevent a user, and allow google, what prevents the user from going to google to retrieve the image (the later without your knowledge)?
The simple solution is to list google (and other major SE's) in your robots.txt, and specifically in your image directories.
Unless your a photographer selling images, there is NOT any benefit to offering your images to SE's.
Visitors whom are looking for specifically named images could care less about the other content on your website (s), which results in no benefit for serving the image.