with iCloud Photo Library, Apple provides you the option to always keep high-resolution copies locally.
To be sure the ability of a complete backup, you have to anoint one computer as the place you set Download Originals to This Mac in Photos (Preferences > iCloud). There is also a possibility to set that option in iOS.
If that option is set on none of your Macs, iPhones, or iPads, you’ll wind up with a hotchpotch of preview image and videos and their originals scattered among devices, with the “truth”—the high-resolution media—only stored for sure in iCloud. That’s problematic if you’re concerned Apple might have a server-farm meltdown and lose your originals (seemingly unlikely), a glitch that doesn’t affect everyone but just a handful of people (a high possibility but statistically unlikely for any given person), or you want to stop paying for iCloud storage in the future and switch to, say, Amazon or Google.
I’ve picked a master-copy machine: a Mac with a multi-terabyte external drive where I’ve located my Photos library. It’s backed up continuously to a local external drive, as well as to a cloud-hosted backup service. In the worst case, I should have two cloud versions of everything: my backup service and Apple’s iCloud. In the best case, I can just restore from the local backup.
Recommend: if you would like to backup your iPhone/iPad/iPod to Mac before jailbreaking, you can just simply have a try Syncios Data Transfer for Mac. To avoid losing important data, it is very necessary to backup all important files to your Mac so that you can easily get them back whenever you lost them.