Picasa 3’s Sync functionality let you effortlessly keep your uploaded photos and videos up-to-date to the web in order for your changes to be reflected online. Now with a click of a button, you can enable syncing of any album, whenever you make a change to a photo or video in Picasa (that includes edits, captions, tags and geotags) your online copy of the album will be seamlessly updated as soon as you log in. If there are some photos that you'd rather not share, you can always suppress those photos from appearing in your album.
The sync feature also allows you to sync starred photos only -- so if you use stars to single out the "cream of the crop" in your photo collection, this can make sharing your very best shots easy. Use the drop-down menu next to the 'Sync to Web' and 'Share' buttons to enable starred-photos-only syncing, or specify other sync preferences, such as upload resolution, etc.
Source:→ Google Photo Blog