Applying the star icon
You can add stars to your photos in Picasa to mark your favorites. First select all the photos in a folder or album in Library view that you'd like to star. Then click the Star button, located just to the right of the Photo Tray. You'll see a small star icon appear on the thumbnail images of the starred photos.
Once you've starred your favorite images, you can easily locate them by using the star search filter located to the left of the search box at the top of your Picasa Library. If you've uploaded a particular folder or album to Picasa Web Albums and have enabled the Sync feature as well, you can choose to sync only starred photos. Learn more about the Sync to Web feature.