In iOS 12, how to select all photos in Camera Roll?

iCloud is a syncing service, not an archiving service.

Really? I just visited iCloud - Apple, which says this:

Take all the photos you want. Without worrying about space on your devices.

To save space on your devices, the original, full‑resolution photos you take are automatically uploaded to iCloud. You’ll still always have a lightweight version of every photo on your device, and you can download the originals whenever you need them.

My family member with the iPhone is just a little nervous about activating Photos in iCloud because he wants to make sure he doesn't accidentally end up with low-res versions of photos when he needs the full-res version. I gather from the above on apple.com that if he turns on Photos in iCloud, it will move all of the full-res versions of his images to the cloud, and replace them on his phone with low-res versions? Then if he wants an archive of the full-res versions on his Windows system, he can visit the iCloud website from his Windows PC and download all of the full-res files?

In theory, it sounds like it should meet his needs, but as I said, he's a little nervous. For example, this week he tried transferring his ~4,000 images from his iPhone to my Mac via Image Capture, and it errored out after doing the first few hundred. If, say, all of his full-res images went to the cloud and got replaced by low-res images on his iPhone, it would be really, really awful if any error or glitch prevented him at that point from downloading all the full-res images to his Windows system.

I just want to make sure that whatever transfer path we use, it's absolutely bulletproof and absolutely protects his irreplaceable full-res image archive.

Thanks. One wrinkle is that the iPhone owner uses a different iCloud account than I do on my Mac. However, the ultimate destination for the files is the phone owner's Windows PC. I see that iCloud for Windows exists, so I'll suggest that he try installing it on his Windows system and see if he can make it work that way.

Incidentally, I noticed when we did the trial transfer by Airdrop from the phone to Mac, the still image files remained in HEIC format. Whereas when we've transferred still images via Image Capture they get converted to jpeg. Do you happen to know what format they would appear in if uploaded to iCloud from a phone, and downloaded from iCloud by either a Mac or Windows system? He wants to end up with discrete files that he can move around in various folders, open in Adobe Photoshop, etc.

Thanks, we'll check it out.

On a related topic, when we turn on Photos in iCloud on the phone, does that cause two copies of each photo to exist -- one remaining on the phone, and one in the cloud? If we then delete a photo on the phone, what happens to the copy on iCloud?