You can help protect your images for copyright reasons by adding a watermark to your photos. A watermark in Picasa is a small text that's added to the lower-right corner of photos during the export process. The text can be your name or business name, etc. If you want a larger watermark, you could use the Text tool, for example, showing the outline of text only.
You can add a watermark to a photo by using the Export feature of Picasa:
Select the photos you'd like to export.
Click the Export button in the Photo Tray.
Select the 'Add a watermark' checkbox.
In the watermark box, enter your watermark text.
Click OK. Your photos will be exported with your watermark in place.
Picasa will never make changes to your original photo file. Exporting pictures with edits or watermarks applied creates an edited JPG copy of the original photo.
Note: Under Tools > Options > Web Albums, the watermark feature no longer worked as of 2016 when Google got rid of Picasa Web Albums online.