Images are very rampant online that you can’t tell whether someone is using their own images or just “borrowing” these images from its direct creators. If you are one of those people that are annoyed by how bloggers use your images or you want to make your personal pictures become invisible online, you can do so by preventing Google from indexing your page using either the meta keyword or editing your Robots.txt file.
With these codes, your images are expected to not appear not only on Google but in other search engines as well. This does not cover the images that were manually grabbed by your visitors in your blog.
Read how it’s done here: http://wphacks.com/how-to-prevent-google-from-indexing-your-images/