Icloud backup

Set up and use iCloud Photos - Apple Support

Make sure to understand that this is not a backup solution.. iCloud Photos will sync your photos across your devices.

There is an optimize storage switch you can flip that should save you space on your device by using lower resolution photos on your phone.

Moving your photos to iCloud only makes them available to all your iCloud devices.

If you delete them from the device you also remove the iCloud copy.