What happens to my iCloud photo storage, and corresponding photo storage if I delete images off my iPhone? [duplicate]
iCloud Photo Library synchronises a library of photos between all your devices. The source of truth is the cloud, so deleting a photo on your phone connected to iCloud Photo Library will synchronise to all your other devices.
The right way to free space on your phone while keeping it connected to iCloud Photo Library is to turn on Optimise iPhone Storage in Settings → Photos.