I want to move my old pictures to iCloud to free up iPhone storage space (iPhone already at full capacity)
iCloud is a syncing service, so any photos you delete on the phone will also get deleted in iCloud. This is obviously not what you want.
You can configure the phone to automatically manage local storage though by opening Settings, scrolling down to Photos and selecting the &quot;Optimize iPhone storage&quot; option. This will store the full resolution photos in iCloud while just keeping a low-res version on the phone itself. If required (for editing or sharing) the full-res version will get downloaded again automatically.
See Apple's support article for more details on how to setup and use iCloud Photos.