I use a digital watermark-- it is embedded in the image and cannot be seen but it allows me to track, to trace, and to identify my images no matter where they end up. It also allows me to self-destruct the images if I decide I want to do that. Pretty cool stuff.
Unfortunately, I can't turn you guys on to this app.-- it's not available to the public yet.... but there are other apps that can covertly protect your images.... digimarc embedded watermarks can warn against copying and printing as well as restricted use and adult content-- and I'm told that digimarc embeds can be traced as well ( although where do you find the software?). I think they can also be set to degrade the image file over time. At the very least, a digimarc embedded watermark is a way to wholly identify an image as your own-- which allows to you exert legal power over any entity that uses that image without permission. I used to use a digimarc watermark to great effect-- it helped me to get paid for several "stolen" photos. If you have Photoshop-- you have to ability to add digimarc watermarks.
I also include a tag too... with my name on it. This lets people know that the image belongs to me... if they decide to rip my image anyway, well... they've been warned ( in effect ) and so I don't feel so bad when I take action against them.
I might suggest too that some of you consider a text overlay.... put your name right across the whole image and reduce the transparency until it is barely visible. This sort of text "watermark" is really hard to remove in most cases.
I'll add an example of a photo with a transparent text watermark ( or two ), a tag, and an embedded digimarc watermark ( see if you can read the "invisible" watermark using Photoshop-- it will read "Imagebridge Demo, copyright 2008, restricted, do not copy"-- if I was still registered with digimarc I could put my name and all that jazz in there as well ).