No. If you delete from your phone, they won't be included in future backups and will be lost.
You may want to consider turning on iCloud Photos and its Optimize iPhone Storage option: Set up and use iCloud Photos - Apple Support
Note that iCloud Backup and syncing to iCloud Photos are two different things but if you sync to iCloud Photos the photos will not also be included in future iCloud Backups.
iCloud Photos is a syncing service. Any photo you delete from a device where iCloud Photos is turned on will also be deleted from iCloud and from any other device connected to iCloud Photos. You can potentially reduce the storage space required on a device by turning on Optimize Storage.
Apple does not currently offer cloud-only storage for photos. If that is what you want or need, you may want to explore 3rd party options such as Google Photos or Dropbox.