Picasa Web Albums has now become private in ‘real’ sense. Previously, you could either set album public for everyone to view or mark them unlisted. When marked ‘unlisted’ album can be still accessed if direct access URL is known to any person.
Now there is new (3rd) option ‘Require sign-in to view’ to keep your Picasa web albums private. Go to album properties and select “Require sign-in to view”, only the people you are explicitly sharingÂ the album can see it.
Aa pointed by GOS blog, to differentiate albums on the basis of access, albums marked private will have ‘sign-in required to view’ label below them (as seen in image). This is very good move from Google in attempt to make Picasa more private place for personal images.