You'll have to make your own icon for each user or use a URL provided by
someone else. This is not an automatic process like it was on bN.com, just
a user-side script. Nobody else will see the icons you set for them unless
you share a screenshot.

16 x 150 image resolution seems most reasonable for small and wide viewports.
There are no actual limits on image sizes but for the sake of keeping the
headers readable, I'm just going to suggest that as a standard size.

I'm using a lazy CSS trick to load a background image into .ch, the comment
header. This means any text will draw on top of it instead of staying clear
and giving it its own space.

As a result... the image has to be centered otherwise it draws behind whatever
is to the left (username, date) and the right (vote options, score)

Someone likely can do a much more elegant injection, something beyond my skill
level, if they so desire to revive Nabbit icons for zBn.

What I did here was simply a proof of concept.

To make it easier for anyone who wants to "tag" users I've made this Stylish
sheet one can edit.