(*) I'm sure I've bumped into some form of the following idea a few times, but, I've been thinking for a while now about a way to embed easy-to-use client-side encryption/decryption into the PM system.
I'd love to see that implemented!
I think adding a native, no-signup-required, encrypted/ephemeral chat feature where people could establish public (or private) "rooms" to have genuine (unincentivized, unlike a lot of posting) and short-feedback-loop conversations would be very feasible (and seriously cool).
That would be a great replacement for all the people who ask me to use Discord or Telegram.
(I think either "Never" or "Always (subject only)" would make a better default for newly-registered accounts than "Always (with message)".)
I don't know what the current default is, but if a user doesn't log again in after creating an account, a PM notification would be the only way to reach him.
(You can even de-duplicate similar images, by defining the hashing to happen over something other than the raw bytes, like, say, over the image data after it's been moved into a consistent colorspace and simplified in various ways. But, I'm unlikely to pursue that. Fun to think about, though.)
Inaccurate solution: create a thumbnail with fixed width, subtract the images. If the resulting image is 99% black, they're probably similar. But this requires far too many checks for large numbers of images.