Documenting this here in order to save someone else time when/if they add their own new/custom topic icons to Forums. You know like the asterix (sic), question mark, idea, etc.
You can add new topic icons in Admin under forums. (Checking the Global box will add to all forums, you don't need to check every box.)
As soon as you add them they will appear in New Posts, and can be selected, so all looks good.
But when the Forums page is displayed you'll get no icon shown, not even the default (blank) spacer.gif.
Issue is that sometime since this was all written, the topic icons have been cached to save time. So you'll need to delete the cache/Forums_topic_icons.php file and let DF recreate it to include the new icon.