Did you save the photos in the Photo App on your Mac and you started using iCloud Photos?
In that case, removing the photos from your iPhone will also remove them from all other devices that are connected to iCloud and have iCloud Photos enabled.
Or did you copy the photos from your iPhone to your Mac, without using iCloud Photos?
One more thing about backups on a computer, worth to mention as well.
There is only one backup saved at a time, unless you use one of the backups to restore from, or archive the recent one. After that macOS will create a completely new backup, which will then be overwritten by the following backup.