How do I delete photos just from my iPhone not my iCloud Photo Library

I have iCloud Photo Library enabled on my iPhone and it is set to be optimised on my iPhone.

How can I delete photos off my iPhone OR is this optimise feature going to do this for me, to free up space on my iPhone?

Unfortunately, Apple does not provide that information when you are about to delete a picture or albums. It would be nice if Apple would give us a Help option in the photos app that can also be removed if desired. Furthermore, I am not an expert on this subject but I will provide a sensible answer from my research, rather than making a comment that DOES NOT answer the question. But Hey, everybody is entitled to there opinions and comments.

So here it goes:

How do you eliminate a photo off your iPhone without eliminating it completely from my iCloud Photo Library and other devices?

You can’t, and it’s a little counter-intuitive as to why. You have to twist your head around to how Apple thinks of cloud storage for media. When you use iCloud Photo Library, the “truth” of your library is in the cloud—that means that the accurate, full, current state of all your images and other media are always stored on Apple’s servers, and the various places you can download or view images (Web, iOS, and OS X) are endpoints. (The only point in which that’s not true is when you have images that remain to be uploaded from an endpoint.)

Thus, if you delete an image in any associated app, you’re telling iCloud Photo Library to remove that image from its “truth,” which means to delete it from the central repository and sync that change to all endpoints. You can choose to only store optimized (low-resolution) media in iOS and OS X, but you can’t delete images in one place and expect them to remain elsewhere. And you can’t maintain separate local albums in Photos that aren’t synced. (In OS X, you can use alternate, non-synced libraries, however, choosing them when you launch Photos. In iOS, you can use alternate photo apps that can have their own libraries, too.)

If you want the ability to delete images from an iPhone or any device without deleting it from all your devices, you have to disable iCloud Photo Library and use another form of sync or another service, like Dropbox, Google Photos, or Amazon Cloud Drive.

https://www.macworld.com/article/3058175/photography/the-problem-with-deleting-photos-from-your-device-with-icloud-photo-library-enabled.html

The optimization feature is supposed to free up space automatically by keeping low res versions of your pictures.

From the Apple support site:

If you turn on Optimize [device] Storage, iCloud Photo Library will automatically manage the size of your library on your device, so you can make the most of your device's storage and access more photos than ever. All of your original, full-resolution photos and videos are stored in iCloud while device-size versions are kept on your device. You can download the original photos and videos over Wi-Fi or cellular when you need them.