Google’s Picasa Web Albums Data API is the approach one can take to programmatically play with content on the Google Photos. Be it browsing the content of the album or upload a new photo from the mobile application, all has to be done through this API if not preferred to be done through the Google’s native apps or web browser.
Google does have documentation and Java developer guide on their Picasa Web API, but the documentation is definitely not self-sufficient to get started with. One would still end up with spending a lot of time to perform research on how to get the first API call working. Once figured out with the very first one, its pretty easy to get going with rest of the API calls.
It is important to note that since they have retired Picasa, most of the mutation operations have been removed from the API. These include creation, updation and removal of albums, updation and removal of photos, support for tags, comments, contacts and featured photos. Enhanced information on removed features are available in their release notes.
Considering the removed features, the important operations we are left with include listing the albums and their contents and uploading photos and videos. If you are keen to get started with API access, you can consume their libraries which internally takes care of various aspects of the entire flow including serialization / deserialization of requests and responses, invoking the HTTP call, authentication etc. If you are interested to implement all these operations for performance or any other reasons, you can still inspect the default implementation to analyze the request and responses and mimic the same in your custom client code.
To learn more and start playing with the API, head on Google Picasa Web Albums sample implementation that is available.