You might want to start by reading this article
Change your iCloud feature settings [iOS and Mac] - Change which apps sync and store data with iCloud - Apple Support - "'When you turn off an iCloud feature on a device, information stored only in iCloud for that feature is no longer available on the device. If you turn off Mail, Contacts, Calendars, Notes, Reminders, or iCloud Photos on just one device, you can still view and change your information on iCloud.com and on another device you set up for that feature."
You would need to disable your devices for iCloud Photos. Then you can delete them from iCloud using a web browser interface to iCloud.com and the Photos app on the iCloud web site.
Make sure you device an alternative strategy for backing up photos such as to a computer. This support article outlines the various ways you can back up your iPhone, iPad, and iPod touch --> How to back up your iPhone, iPad, and iPod touch - Apple Support