Just a quick explanation, in case anybody else is wondering about missing avatars.
It’s a Discourse problem, caused by circumstances outwith their control, and they are working to fix it as soon as possible. It only affects the system default letter avatars.
I thought I’d post this so that others don’t need to go through the “is it me or is it them?” debate I’ve just been having with myself: it’s us, not you! 
