[Editor's Note: Per provider, this API will be deprecated in January 2019. Developers are encouraged to migrate to the Google Photos Library API]
From their site: The Picasa Web Albums data API allows client applications to view and update Picasa Web Albums content in the form of Google data API GData feeds. Your client application can request a list of user albums, photos, comments, or tags, edit or delete content in an existing entry, and query the content in an existing entry.
In January 2019, Google is deprecating its Picasa Web Albums Data API and disabling all associated OAuth scopes. Any existing applications using the API will continue to work until that time. In March 2019, the Picasa Web Albums API will be turned off at which point all calls will fail.