my answer, with picasa from google, here's what you need to do.
- download and install picasa
- launch picasa and let it start indexing your pictures
- find the picture you want to add the watermark to in picasa
- double-click on the picture
- the picture will then come up in a photo editing tool where you can edit your picture
- once you have your picture the way you want it, click on export at the bottom of the screen
- take note of where picasa is going to export your file to (the file name will stay the same, but it will be in a different directory, so it won't change your original picture)
- at the bottom of the export screen, click on the add a watermark checkbox.
- enter the text you want displayed in the watermark in the add a watermark field
- click on ok