I don't know if it would help much in email, but what some online "art galleries" do is apply an obnoxious watermark to thumbnails or preview images. So the person can't get at the original, un-altered image unless they purchase and download it. Not so much a preventive as a "discouragement" of copying images.
I saw this at something called E-Zine. It's more about Web sites than e-mails, but might still be useful:Prevent Images From Being Copied on Your Websitehttp://ezinearticles.com/?Prevent-Images-From-Being-Copied-on-Your-Website&id=1874489