How to keep my backup photos on iCloud without being deleted when I only delete photos on my iPhone device?

I now have all my photos backuped on iCloud, both as a backup and for all individual photos. My other iPhone also on the same iCloud account is now seeing those photos too.

It seems that if I take photos on my iPhone, that photo will stay on my device as well as auto-sync to my iCloud. How about deleting a photo or a bunch of it on my device so to save space? Do my back up photos on iCloud be deleted as well? How can I avoid this to save my backup photos?

Thanks a lot!