The camera roll's defaults display images chronologically based on the image's timestamp. I've got thousands of photos on my phone going back years.

If you hack my phone and plant some photos with a sufficiently old timestamp I'd never notice them. I can't imagine my situation is all that uncommon either.

A couple gigabytes is enough to ruin someone’s day but not a lot to surreptitiously transfer, it’s literally seconds. Just backdate them and they may very well go unnoticed.

Gigs of software updates and podcast episodes are regularly downloaded to phones without being noticed.

How frequently do most people look at their camera roll? I'd be surprised if it's more than a few times a week on average.

Does an attacker even need access to the phone? If iCloud is syncing your photos, your phone will eventually see all your pictures. Unless I've misunderstood how this works, the attacker only needs access to your iCloud account.

As others have said, people have a lot of photos. It wouldn't be too hard to hide them a bit from obvious view. As well, I rarely look at my gallery unless I need to. I just add a few photos occasionally. So maybe once every two weeks I look at my gallery, plenty of time to initiate that.