delete all the photos from icloud
I don't know how to delete the pictures stored in icloud without deleting the photos on my iphone.
It should be as easy as select the fotos in icloud.com and delete, but it aint.
iMac 27″, macOS 10.15
Sorry, but iCloud Photos is syncing all photos to all connected devices and if you change anything, those changes will be synced to all devices.
However, transferring your iCloud photos to your computer and deleting them afterwards could be an option.
This article has more details about that:
Archive or make copies of the information you store in iCloud - Apple Support