Although Picasa does not support image-file watermarks, such as logos, the photo management application from Google lets you add custom text watermarks to your photos when exporting them either to Google+ Photos or to your hard drive.
Export Watermarked Photos to Google+ Photos
Launch the program and log in to your Google account. Open the Tools menu and select Options.
Open the Google+ Photos tab and tick the check box next to Add a Watermark for All Photo Uploads.
Enter your custom text watermark in the text field and click OK. Upload your watermarked photos to Google+ Photos by selecting them and clicking the Share on Google+ button.
Export Watermarked Photos to Your Hard Drive
Select the photos you want to export to your hard drive from the library and click the Export button.
Tick the check box next to Add Watermark and enter your custom text watermark in the text field. Click the Export button to export the images to your hard drive.