For signal then the issue becomes saving who owns what image (so that you can re-issue “passwords”) and THAT is much more dangerous to the users than simply allowing users to grab semi-anonymous links into their cdn with enough of a url to be nearly impossible to iterate through every combination without hitting tons of rate limits. (Ignoring this location cache timing issue.)

Edit: Actually... (in signal's case) it might be possible to provide the user's device 2 tokens, 1 to access the url and 1 to issue new access links. Then the user can request a new access link with their second token when their url access token expires. Signatures would help prevent it from needing to be stored in the database. It would be interesting to try.

Edit2: Also I am now curious... does this mean only text messages are e2ee? yikes.