delete photos from iphone and not icloud

You would have to turn off iCloud Photos on your iPhone. The purpose of iCloud Photos is to sync the photos on your devices so they are exactly the same. When you delete from one, it will be removed from iCloud and any other device that has iCloud turned on. If you have no devices that have iCloud Photos turned on, then the photos there will only remain for 30 days, so if you turn it off on your phone, make sure you either have another device using it or you turn it back on within 30 days.

As Mac Jim ID said, if you turn off iCloud on your phone and delete pictures, then they won't be deleted at iCloud.com. But if you ever turn on iCloud on your phone again, then those pictures will be transferred back to your phone. iCloud Photos ensures that the pictures on your phone will be exactly the same as those at iCloud Photos!