The short answer
With iCloud Photos on, you can't delete a photo from just the iPhone, because both hold one library and a delete syncs everywhere. To free space but keep everything in iCloud, turn on Optimize iPhone Storage. To have a separate library on the phone, turn off iCloud Photos on that iPhone. To keep photos outside iCloud entirely, copy them to a computer or another service first.
This is one of the most searched iPhone photo questions, and the answer surprises a lot of people: with iCloud Photos on, there's no such thing as "delete from iPhone only".
Why deletes sync everywhere
iCloud Photos isn't a backup. It keeps one library in step across your iPhone, iPad, Mac and iCloud.com. Add a photo on one, and it appears on all of them. Delete it on one, and it's deleted on all of them.
Deleted photos go to Recently Deleted on every device for 30 days, then they're gone from iCloud as well.
So the question becomes: what do you actually want?
"I want the space back, and everything kept in iCloud"
Turn on Optimize iPhone Storage.
- Open Settings and tap your name.
- Tap iCloud, then Photos.
- Choose Optimize iPhone Storage.
Your full-size originals stay in iCloud. The iPhone keeps a smaller version of each photo and downloads the full one when you open or share it. The phone does this gradually, and mostly when space is getting low, so don't expect gigabytes back in the first minute.
This needs an iCloud plan big enough for your whole library. And it only works if you have a signal: a photo whose full version isn't on the phone takes a moment to load.
"I want a separate library on the phone"
Turn off iCloud Photos on that iPhone. In Settings › your name › iCloud › Photos, turn off the switch for syncing this iPhone.
iOS asks whether to download the originals to the phone or remove the iCloud photos from it. After that, the iPhone's library and iCloud's are separate: you can delete on the phone and iCloud keeps its copy.
The catch: new photos from this iPhone no longer go to iCloud either. You'll need another backup for them.
"I want them off my phone and out of iCloud, but not lost"
Copy them somewhere outside iCloud first, then delete as normal.
- To a Mac: Image Capture or the Photos app can import from the iPhone over a cable. If the Mac's Photos library also uses iCloud Photos, it's the same library, so export the originals to a folder instead.
- To a Windows PC: the Windows Photos app imports from an iPhone over a cable.
- To an external drive: select the photos in Photos, tap the share button, choose Save to Files, and pick a drive plugged into the iPhone.
- To another cloud service: use its app to back up your camera roll, and check the upload finished before deleting anything.
Deleting what you don't need anywhere
Often the simplest fix for a full phone and a full iCloud plan is the same: a lot of the library is duplicates, blurry shots, screenshots and videos you'll never watch. Deleting those frees space on both at once, and every delete still waits in Recently Deleted for 30 days. Our guide to freeing up iPhone storage is a good place to start.