If you have iCloud Photos enabled, then your photo library is in iCloud, and copies of the photos are cached locally. In this case, higher-resolution copies are cached locally.
A common source of consternation: If you delete photos locally with iCloud Photos enabled, the photos are deleted everywhere.
As for safety, here is how to: Archive or make copies of the information you store in iCloud - Apple Support
Downloading originals as configured here also means you can export and archive those full resolution photos locally.