Is there a more intuitive way to add an attachment from GMail (either standard or Inbox) to Google Photos than saving it into the Google Photos folder in Google Drive?
Edit 2019: This answer is now out of date.
Google Photos no longer synchronizes with Google Drive. See the official blog post.
Now you just need to save these attachments to Google Photos in Google Drive. Voila! Your images are automatically saved to Google Photos
Login to your Google Drive account. Then press preferences button. You will see 'Create A Folder For Google Photos'. Check it enable. And now go to your Gmail account and open a mail including photos. Add it to Google Drive first. Then press organize button and move the photo to new Google Photos folder.