PicMarkr is a free online program enables you to affix a "digital watermark" (text or an image) as a security feature on your photographs. For example, you could affix a copyright, business logo, or "DO NOT COPY."
PicMarkr works with Flickr, Picasa, Facebook, and images stored on your computer. Find beta http://picmarkr.com/index.php
What applications do you use to protect your image files? Let us know in comments!