It reminds me that I've been very slowly experimenting with an idea I've been calling "avatar auth" for a Secure Scuttlebutt-like P2P social media (or possibly just directly with SSB). The idea is that the raw original image file used to build an avatar acts as the hash/input to a key derivation function so that the raw avatar image itself is the "password" to the account. You force a lossy series of steps between the raw image and the final avatar (crop, shrink, corner rounding, maybe some simple filters) so someone can't just copy an avatar as they see it on the service itself to log in. You can't entirely eliminate "copy-cat" avatars (but they'd have different keys/account numbers), and you have some difficulties with avatar migration, but overall it's a really simple login system idea (choose your avatar image file and go).