Dropbox is known to have intergration with PhotoDNA, a state program that matches file signature with a database of known illicit images. This would be impossible if the encryption used is irreversible and as far as I know dropbox never claimed otherwise.

Couldn't dropbox hash the images on the client, and upload the encrypted images as well as the hash? No need to upload an unencrypted file to do matching.

Depends on what kind of hashing they do: a scheme like yours using a cryptographic hash would be defeated by just randomly changing a single bit in the image (eg appending some garbage bytes at the end); or re-encoding the jpeg. Of course, it could still catch non-techy people.

I'm not claiming it is end-to-end, nor did I expect it to be, but perhaps some people would assume that. I thought you were claiming they don't use encryption at all.

It's effectively nothing when Dropbox themselves have access to all your files whenever they want.

And whoever hacks dropbox, yours or all of them.

They do, but Dropbox controls the key and their services can decrypt customer data, so the value of the encryption is limited at best.