You can add copyright information to Photoshop images and notify users that an image is copyright-protected via a digital watermark that uses Digimarc PictureMarc technology. The watermark—a digital code added as noise to the image—is generally imperceptible to the human eye. The watermark is durable in both digital and printed forms, surviving typical image edits and file format conversions—and is still detectable when the image is printed and then scanned back into a computer.

......For more detailed information on embedding Digimarc watermarks, refer to the Digimarc Watermarking Guide, downloadable for free from the Digimarc Web site at www.digimarc.com.

This is probably part of the later version distributions as a plug-in.

The worst case scenerio I might be able to eliminate the letters but I prefer not to unless there is no choice.

One thing to be aware of is I've seen claims that digimarc can survive resampling, not sure how accurate that is and I'd imagine it really depends on how much you alter the image. At some point the embedded data is going to become unreadable no matter what you use. The only question is how much alteration is needed.

One alternative you could do is use EXIF data, this is easily erased. Simply saving for the web in most applications will do that but if you're trying to stop amateurs some of them might not even be aware of it.