That means your photos are backed up online somewhere. If they are synced and you delete them from the phone, they get put back on the phone....or just a thumbnail. You need to delete from both.
Are they syncing with OneDrive or other cloud service? If they are, you have to delete them from the sync location, which will remove it both from the cloud drive and your phone. If you are backing up to Google, it's not the same as sync. Backing up means that a copy of the photos will remain in the cloud storage even if deleted from your phone. Syncing means that if you remove it from your phone, it will come back when your cloud and phone has it's next sync cycle. If you want it removed from both places, you have to remove it from the cloud side. Google backs up (not syncs), so removing it from your phone will preserve a copy in your Google account, but it will not come back to your phone unless you specifically download it, however if you remove it from Google backup, it's gone forever. You can turn off Google backup and you can turn off OneDrive sync., but then you will have no backup copies at all unless you manually transfer them to an external drive.
Agai, because you are syncing them somewhere, not just backing them up. You must unsync or delete dir tly from the synced drive ir they eill keep coming back.
