Here is how to watermark a photo with GIMP (GNU Image Manipulaton Program), a free and powerful yet easy to use photo editor. I base this on the same picture jzzr used in his Photoshop tutorial available here:
http://www.facialforum.net/showthread.php?t=36238
The first step is to go to g i m p . o r g and download the latest version.
The rest is pretty standard. File>open in the upper left of the main window and browse to the file. All tools are in the "Toolbox" to the left. All layer and selection options are on the right hand side. Click on the big letter "A" in the toolbox for text, then click on the picture where you want to add it. Use the Move and Resize tools to do just that, move and resize. The highlight the text layer in the right hand window and use the slider to set the opacity. I set it at 40% but that is completely a matter of preference. The just click File> Save As and save with a new name. That easy.