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If you’re using iCloud photo, anything you delete off your device will be deleted in iCloud photo as well. iCloud photo is a synchronization service, not independent storage. And if using iCloud photo, then your photos are not included in your iCloud backup.
If not using iCloud Photo, the photos are included in your iCloud backup. But the very next iCloud backup will overwrite the last one, so that will wipe out your photos there as well.
The only way to safely delete photos from your device is to first copy them to some independent storage space - a laptop or desktop hard drive, an external hard drive, archival online storage (e.g. Dropbox, Box, etc). Once they are safely transferred somewhere then they can be deleted from the device.