Face it, the Internet harbors far too many low down dirty image thieves. While certainly not always effective, adding a watermark to photos will help reduce theft. And if something is stolen, it can make it easier to track.
How to Watermark Your Photos
Watermark generally means a faint marking that you can see through. Originally from paper being marked with water in specific designs. Now, in general use, it just means adding text of some sort to your photos to brand them as yours. Many programs today have text tools that let you type directly onto a photo. You can also create a brush for programs like Photoshop and Photoshop Elements from your logo for quick application to multiple files. One photographer friend of mine uses pale lines across her photos, especially those on Facebook where sharing is rampant, to ensure they are obviously used without permission. For almost all programs you will create a new layer, and type text onto the photo. There is a list of links to program specific instructions at the end of this post.
Where to Put the Watermark
Put the watermark somewhere on the photo where it will be difficult to simply recrop the photo. Secondly, if you can, place it in a complex part of the photo to make it more difficult to clone out. This is why large watermark patterns work so well, they are a pain to clone out of the image.