Play_Bach - 12:48 pm on Mar 29, 2012 (gmt 0)
You don't have to watermark what's show on your site.
Put images in 2 directories called /site and /off-site and create a rewrite rule that redirects anything requesting the same image that isn't coming from your pages, to load them from the off-site directory.
Good advice and worth considering. Implementing it might go a long way towards stemming the tide. Seriously, what do you have to lose?