If you create albums with Picasa Web Albums, you must know that your album's URL will be accessible to everyone. If you don't make the album public, it will not be listed in your gallery, but can be accessed if someone guesses the URL.
"What's the difference between public albums and those that are unlisted?
When you upload photos to Picasa Web Albums, you can mark your albums as public or unlisted.
* Public albums appear on your "My Photos" and "My Public Gallery" pages. These albums can be viewed by anyone who knows the URL for your public gallery. This URL is easy to share – it's short and based on your Gmail username: http://picasaweb.google.com/GmailUsername
* Unlisted albums do not appear in your public gallery; only you can see them on your My Photos page. Since they don't appear publicly, they're accessible much like an unlisted phone number – anyone who knows the specific URL of an album can view its contents, but there's no directory for finding them."
(From Picasa Web Albums FAQ)
Philipp Lenssen even found an album of Larry Page that's not accessible from his public gallery.