Welcome to JustAnswer, Elliot
Give me a moment, I will share the instruction to do this.
Have you created account in Icloud.
That's good. Now you need to only do few thing as mentioned in below document.
Also make sure you have enabled backup as shown in below image
Once you enable Icloud and turn your backup on. Allow it to sync all the photos to your Icloud account. Once done you can check if the photos are properly backed up. Then try to delete it couple of pics from your phone and retrive it back from icloud storage
I would recommend you to use dropbox to take backup of your phones and free your Iphone's space. your photos will still be there in your dropbox account
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Your devices use iCloud to back up your photo, and they are in sync, so deleting from a device deletes fro mthe iCloud storage. However there is a new space saving feature in iCloud Photo Library called Optimize Storage.
All devices will need iOS 8.3 or later.
Then on each device turn on iCloud Photo Library.
Tap Settings > iCloud > Photos, then turn on iCloud Photo Library.
Then set it to Optimize Storage.
You can do the same with your Photos Library on your Mac.
This lets the software manage the storage on your device, making optimized versions of photos and videos to store locally on your device to save space. The full resolution versions remain in your iCloud backup.
You can learn more about this feature here:
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