Updated April 2019.
Protecting one's images and text online is an important topic, and one I tackled at my Chicago Creative Expo WordPress talk in March.
What makes this most relevant is the story of Noam Galai (see it on The Stolen Scream), whose image (a screaming self portrait) has been stolen countless times across the globe. He even submitted it to Getty Images, only to be rejected, but a copy was being sold on its site! Frankly, you can't be too careful.
No system is foolproof, but there are steps you can take to better protect your content.
Copyright and protecting your content
One of my favorites for searching down copies — and a fun one at that — is TinEye. Taking a Google photo of Michael Jackson as an example, I did a search for similar images. The results were thousands of the same photo plus those "most changed." This isn't surprising for a celebrity photo, but if you're concerned your images are being reproduced without your permission, it's a fast and amazing engine.
Next up is WP-Copyright Protection. For most browsers, it disables text and image copying (i.e., no selecting or right-clicking) and keeps your site out of an iframe (another way to grab content). Sort of an all-in-one, it's effective and lightweight. It's my favorite free plugin to prevent right-click downloads and copying of text.
Copyright Proof says it has "teeth" and only protects your posts with a digitally signed, time-stamped copyright notice (if you don't claim authorship, it's not yours!). It also provides licensing and is more of a deterrent, providing a badge for your site. But, it also provides one free image takedown per year service via the DMCA — otherwise it's a paid service.
Image Watermark allows you to bulk watermark previously uploaded and new images, and disables right-click, among other features. It's just one of many watermark plugins.