icloud photos

Yes, since they are synced using iCloud Photos.

If you need to delete one or more pictures (or other stuff), I suggest that you follow these steps:--> Archive or make copies of the information you store in iCloud – Apple Support

When done (data/media are all safely stored on your computer) you can delete whatever you want.

Regards

Giulio

Yes - iCloud is a sync service (not a backup service). Whatever you do on your phone is done in iCloud and on your other devices.