If you are running out of storage space on your iPhone, you may think that removing some photos is a good way to free up space. But using iCloud Photo Library means that deleting a photo will delete it from all of your devices. The solution is not to try to delete a photo at all. You simply need to turn on Optimize iPhone Storage in your Photos settings. This will allow your iPhone to manage your photos so that only small thumbnails are stored for most photos. The original high-resolution photos will always be in iCloud, and can also be stored on your Mac. Optimizing allows you to get a lot of storage space back, while still having all of your photos available to you. You can do the same for your Mac or second Mac to save space.