Tim via users writes:
At some stage I discovered that the MATE login greeter can read user
icons from "/var/lib/AccountsService/icons/", so I put files in there
for users with the same filename as the username, plus a png suffix.
e.g. Mine is /var/lib/AccountsService/icons/tim.png
All the user icons in that directory are owned by root:root.
I don't recall how I discovered that, nor if I had to do anything else
to make that work (I suspect I didn't, it just looks there).
I also found that one when I was searching for a solution.
We just need to find two more alternative ways of doing this. Then there's a
good chance that at least one out of all the alternatives will work and
it'll finally be Linux's year on the desktop.