In my TagManager I have tags like:

The first one, accessibility, uses an icon I downloaded from, the other one, gym, is a emoji I copied from or I typing on a keyboard with support for emojis.

Hands on!

Create the Icon

First, you have to create a tiddler with similar to this:

The prefix $:/images has to be there. The unicode part of it is for my own understanding.

Eventually, in your filesystem, it creates a file wiki/tiddlers/$__images_unicode_gym.tid like this:

created: 20210802195500933
creator: dmitryrck
modified: 20210802195507116
modifier: dmitryrck
title: $:/images/unicode/gym
type: text/vnd.tiddlywiki


Set the Icon on the Tag

You can either use the TagManager like this:

Or you can open the tiddler of that tag:

If the tiddler of that tag does not exist, just create it.