All of you users with no avatar, go get one so I remember you all please! It’s an accessability aid in a way :p

Frankly, this could be implemented client-side. If there's no avatar set by the remote user, then have the client or Web UI or whatever hash the username/home instance pair and then use the bits of that hash as inputs to procedurally-generate an image. There are a number of forum systems that do this.

https://en.wikipedia.org/wiki/Identicon

An Identicon is a visual representation of a hash value, usually of an IP address, that serves to identify a user of a computer system as an effectively random form of avatar while protecting the user's privacy. The original Identicon was a 9-block graphic, and the representation has been extended to other graphic forms by third parties.

(Note that using an IP as an input to the hash, as is mentioned here, is not really a good idea, since the IPv4 address space is small enough that one can reverse the hash by brute-forcing it to obtain the user's IP address, and the IPv6 address space is only sparsely-used, so I bet that one could do the same there with a bit more work.)