OP is about sending the photos over an unencrypted channel, not that OneDrive unencrypted access to them. Normally when talking to a cloud storage service, the data will be encrypted in transit (and then again when at rest).
Not encrypted != not E2EE
Most online services are not E2EE, but almost all have E and that is expected and industry standard.
Encrypted or not, photographs are typically protected from view and not shared with the world. They are behind an authwall. Imagine if you are on Facebook and you upload a photo album. You share it with a group of friends. Do you expect Facebook to spray the content of those photos all over in public without your consent? No, same with OneDrive; my sharing options specify how I want the files/photos/content to be protected, and if OneDrive decides to frivolously ignore my sharing options, then that's a security incident, encrypted or not.
iCloud have mode to enable E2EE for almost all data called "Advanced Data Protection"
I believe iCloud can be configured to be E2E, but it is not E2E by default.
It is E2E in transit but not everything is encrypted at rest by default. Users now have the option for at rest encryption but you need to make sure that you store your encryption keys properly as there is no net to catch you.