When you're taking pictures with your iPhone device, they will always be at maximum quality. Maximum quality also means a decrease in storage space and having too many pictures taking all the space on your iPhone will eventually cause some performance issues. Deleting photos from the iPhone can be considered as a backup measure if you have any other storage devices (eg. NAS, iCloud, FTP server) as well.
Here I will show you how to take a backup of the current images stored on your iPhone as well as turning off options to have a clean phone, plus access your content from the iCloud through the dedicated app.
Enable photo import to iCloudThis procedure will copy all the photos in your iPhone device and store them in the online cloud. Make sure you are connected to a network (preferably WiFi) that provides fast Internet access. Open Settings > Photos & Camera and enable the iCloud Photo Library. At this point, the procedure will start, and it will take a while until the whole set of images is uploaded. After the images are imported successfully, proceed to the next step below.
Photos & Camera Settings
Create a backup of the images using Photos
This step involves connecting the iPhone to your Mac to import the images.
1. Connect the iPhone to your Mac using the USB cable.
2. Unlock the device using a PIN and accept the confirmation when you're asked to Trust This Computer.
3. At this point, the Photos app will open. If it doesn't, use Spotlight to manually launch it.
4. You can use the Import function available in Photos app to store the images and videos having your device connected to your Mac. Click on Import All New Photos and wait for the process to complete.
5. Once the process completes, a message will ask you if you want to delete or keep the photos on your device. Click on Delete and the images will be removed from the device.
At this point, you have a backup of the images stored on your Mac and in your iCloud. Go back to Settings > Photos & Camera and disable the iCloud Photo Library option. To access the images without storing them on your device, install the iCloud Drive application available on the App Store as shown below, then you can simply open the app and browse through the database of images.
ICloud Drive Installation
Deleting images from your iPhone might be a good thing because it will increase performance in one way or another, and it's always recommended to keep a backup of your images, however, many users prefer to have the images stored on the device as well. It's up to you to decide how you want to proceed, but this is the simplest way to delete photos from iPhone and keep them in iCloud.