Welcome to Android Central! If you're certain your photos have been backed up to Google Photos in the cloud, then open your Google Photos app on the phone, tap Menu, then Free Up Space. This will delete all photos and videos from the phone that Google Photos knows have been backed up. Since they're in your Google Photos cloud, you'll still be able to access them easily using the app, as long as you're connected to the web. You can also re-download any of those photos or videos back to the phone's storage if desired.