I'm first to customize stuff for myself, but I've grown to believe that public-facing customization is best left severely limited when dealing with general audience.
I first realized this after this recent HN comment: https://news.ycombinator.com/item?id=23175151
I agree that too much customization brings the focus on irrelevancies, but no customization (beyond a username) isn't right either. One compromise would be to allow avatars of a certain size but nothing else (signatures, flairs, etc.), but this becomes difficult with threaded comments. Lobsters has tiny avatars but they seem too unobtrusive.
Relatedly, StackExchange has randomly generated avatars, presumably to get around the problem of some people not customizing, but they aren't memorable.
I don't know what would be an improvement for HN. Adding images would ruin the aesthetics and make it worse for people with bad connections (of which we have surprisingly many here; the other day some people were reporting they're browsing HN from the middle of the sea!).
Wrt. customization on forums, I always found avatars to be constrained enough. Signatures were where problems started, because while usually limited in character length, with bbcode/HTML/image embeds, they were of unbounded size - and like GP mentions, often much larger than the comments themselves.
Incidentally, the non-standard capitalization on your username has drawn my eye before, so your username is more recognizable and memorable as-is. I frequently enjoy your posts.
At this point, if I had to design automated avatars for some board, I'd probably download a bunch of these silly Machine Learning image sets with 10 000 images of frogs, or something, scale every picture down to 64x64 or 128x128, and assign one to every user at random, or based on a hash of their username. I'm pretty sure this scheme would yield avatars that have all the benefits of autogenerated ones, and are also infinitely more memorable.