I bought a custom watermark on Etsy, I didn't go back and do the photos I already had up as I would have to go to the original file, mark them, and upload them again. But, if you're just wanting people not to be able to take your photos, I just yesterday found a code to disable right click for your blog.http://www.sosweetdesigns.net/2010/05/right-click-disable.html
If you got to picnik.com, upload the photo you want. Go to create and under the text tab, enter the text you want. You can chose the font, size etc from the little pop up window on the right. Then what you do his move that little "fade" bar until the text is barely visible. I hope this helps. There might be a tutorial on the site to help you through it too.