[ibigb]

From https://support.apple.com/en-us/HT202303 >End-to-end encryption provides the highest level of data security. Your data is protected with a key derived from information unique to your device, combined with your device passcode, which only you know. No one else can access or read this data.

So messages are opaque but photos are not?

[kevinherron]

Read the next line: "These features and their data are transmitted and stored in iCloud using end-to-end encryption:"

Notice the absence of photos from the list that follows?

> So messages are opaque but photos are not?

Yes. Unless you use iCloud backup, in which case your key is included in the backup and technically even your messages could be accessed.