While logical to assume, that's not a given. I think some of us want to believe that the keys to those said images are uploaded encrypted to the cloud. And the ability to sync between devices just means adding more keys to your account. So unless you know something we don't, it's not 100% that they sit unencrypted on Apple's servers, or that they have the keys to decrypt them.
It is sound judgement tho, to assume that they do.
"iCloud content may include email, stored photos, documents, contacts, calendars, bookmarks, Safari Browsing History, Maps Search History, Messages
and iOS device backups. iOS device backups may include photos and videos in the Camera Roll, device settings, app data, iMessage, Business Chat, SMS, and MMS messages and voicemail. All iCloud content data stored by Apple is encrypted at the location of the server. When third-party vendors are used to store data, Apple never gives them the encryption keys.
Apple retains the encryption keys in its U.S. data centers. iCloud content, as it exists in the customer’s account, may be provided in response to a search warrant issued upon a showing of probable cause, or customer consent."
So unless you doubt apples own guide - they maintain the keys and will provide this info in response to govt requests.
They handled requests for 31,000 accounts in the last 6 months based on their reporting and provided data in 85% of those situations.
It is sound judgement tho, to assume that they do.