Grab Google Photos Backup for Mac & Windows, and enable Auto Backup when you install the app on on iOS and Android. You can also manually upload stuff via photos.google.com, but it’s tough to beat the simplicity of letting the robot do the work.
You can also flip a switch to make your Google Drive images show up in Photos: open the Photos settings page, then flip on the Google Drive switch. Everything you store in Drive counts against your Drive quota, regardless of its size.
PS—If you’ve previously installed the backup on Mac, please download a new copy (updated yesterday) as it plays much nicer with the latest networking changes in OS X.