Question: If I switch off iCloud Photos, will my pictures vanish from my iPhone or from iCloud?
A
Answer: No, turning off iCloud Photos on one iPhone doesn't delete anything from iCloud, and your photos stay on your other devices. What it can remove is the copy on that iPhone: if you choose "Remove from iPhone," or if Optimize Storage left only small versions there, those go. Only turning iCloud Photos off for your whole account deletes your library from iCloud, and Apple waits 30 days before it does.