I've been looking for an app that did exactly what this one does for a while. Every other app I've tried (about 30 different apps) all either have a third party 'encrypted drive' that you just have to trust actually does what it says, or they have a pricing model that doesn't match what they offer. ($9.99 - $19.99 a month!?)
Trustful conquers both of those needs. It can protect the photos with zero-access encryption, and uses your own iCloud to sync everything between your devices. It also has a very reasonable pricing model, and the developer was able to help when an issue arose.
When my subscription wouldn't sync to my Mac (after purchasing on my phone), I emailed the developer, and he responded within a few hours before pushing a TestFlight fix for my issue. I am now up and running after only a few days! Great response time and a great app!