If you've opted to backup your photos in Original Quality (which retains the resolution of the image) on Google Photos and are now running low on storage, Google is letting you recover some of that space by converting the full-resolution images to the default High Quality setting via the Google Photos web interface.
Storing full-resolution images counts against your Google Drive storage quota, but compressed high-quality uploads come with unlimited storage. And if you're just looking to free up space on your phone, the latest update to Google Photos now notifies you of images that are already uploaded to the cloud and are utilizing the built-in storage on your phone.
Interested in checking out how much storage you can reclaim? Navigate to Google Photos from the link below and hit the "Recover Storage" button.
Source: Google+ (Google Photos)