True Photos and Videos Backup using Google Photos - Free and Unlimited
Unfortunately, iCloud Photo Library is a syncing service. Any photo you delete from a device where iCloud Photo Library is turned on will also be deleted from iCloud and from any other device connected to that iCloud Photo Library. See: iCloud Photo Library: https://support.apple.com/en-us/HT204264
Apple does not currently offer cloud-only storage/backup for photos. This leads to many people deleting or accidentally deleting their photos and usually end up heart broken loosing beloved photos. I see this numerous times daily on the Apple Discussion Forums.
Your best bet as a true backup for your photos is to use Google Photos.
That's because unlike iCloud Photos, Google Photos is
a) Totally Free.
b) Totally unlimited for storage and backup of photos and videos .
c) A true backup (unlike iCloud Photos Library).
1. If you don't have a google account, you can create one, it's totally free: https://accounts.google.com/SignUp
2. Download Google Photos from the App Store: https://itunes.apple.com/us/app/google-photos-unlimited-photo-and-video-storage/ id962194608?mt=8
3. Launch the App and sign in with your account
4. Make sure the following is configured in its setting then Settings )
• Tap ≡ on top left
• Tap Settings
• Backup & Sync → Backup & Sync = ON
• Backup & Sync →Upload Size = High Quality
5. From now on Google Photos will upload all your device photos to Google Photos. It's faster if the app is open and running, so maybe better for the initial backup of your photos library, but it does this also when the app is not in the foreground, it does this in the background.
6. To free up space on your device do the following:
• Tap ≡ on top left
• Tap Free Up Space
This will delete photos in your photo albums that are already backed up to Google Photos.
You can still view the photos in Google Photos, but you freed up the iPhone storage. If you use iCloud Photo Library, this also frees up your iCloud storage.