Badges, including custom (i.e. user-installed) badges need to be PNG images. You can create your own sets of badges that you can apply to a note’s $Badge. These custom can be used instead of or mixed with app-installed badges. Read more on custom badges If you don’t know where the application support folder for Tinderbox are, don’t worry as you can open the folder in Finder via Tinderbox’s Help menu (last item on that menu).
Note that the name of any badge - built-in or custom - is the name of the PNG file (without the file extension (i.e. you set $Badge to ‘ok’ not ‘ok.png’. This name is used when setting the badge. Note that badge names are case-sensitive ‘ok’ would not match a badge actually called ‘OK’. My understanding is that the names need to be unique across all badge sets. So, if making your own badge set consider giving the items a prefix. you could thus has a OK symbol called ‘e_ok’ which wouldn’t conflict with the built-in ‘ok’ badge.
Badges are stored on your Mac and not in a TBX, so if you open your document on a different Mac (or send it to a colleague) any custom badges not also stored on the other Mac will not be shown.