I am so happy that I finally got rid of Picasa showing in my Gallery. I am so annoyed with it and have been trying to figure out to delete all Picasa albums for several days now. Lucky me, I finally discovered how to go about with it. Getting rid of Picasa in your S3 is so easy, here's how in two steps:
1. Go to SETTINGS › APPLICATIONS MANAGER › ALL TAB › GALLERY
2. Once you're in Gallery, tap CLEAR DATA. It will remove all those synced Picasa albums & photos.
Don't worry, all photos that you've taken with your phone's camera and all downloaded images will not be deleted.
Hope this helps. Problem solved, goodbye Picasa.Ü