Backing up full resolution photos from iPhone while using 'Optimize iPhone Storage' with iCloud Photo Library
They may have updated the iCloud Photo Library for Windows capabilities:


  What you can do with iCloud for Windows
  
  Safely store your photos and videos in iCloud. With iCloud Photo Library, any new photos and videos that you take on your iPhone, iPad, or iPod touch automatically download to your PC. And you can upload new photos and videos from your PC so that you can access them from your other devices too.