I've read a couple of reports of fansshare's images starting to drop out of image search because of the said url tagging.
Nashua the redirect works. Think about whether you really need the watermark. From my understanding in the preview pane, google starts loading the cached thumbnail using javascript whilst it pulls in the full resolution image (which I assume it scales) via a hotlink. The Imaguard plugin intercepts this hotlink and sends back a plain html page. So what happens is that the preview image remains blurry (ie low quality cached thumbnail is used). For my purposes, this is enough of a deterrence for scrapers and it will probably be enough of an incentive to click on the image which of course will lead back to the original post. If scrapers want to lift the low-res preview then so be it. Can't really do much about it and that traffic is useless anyway.
If the user clicks on the view-original, then they are also redirected back to the post via Imaguard.
It's a win win.
This is a free plugin and the developer has done us all a service by developing it. No one is holding a gun to your head saying you must use it.