Just recently, Google has introduced a new interface of their Image Search. From January 25 2013 on, full size images are shown directly inside Google, without sending visit
Hi there here's a new plugin to address this issue on WordPress
https://github.com/mompracem/direct-images-redirect
Instead of using watermarked images, it just redirects the user who tries to access an image directly to the post or page where that image was originally attached to.
It's a new plugin therefore might have some bug, please test and report issues over github thank you