I believe this is a list of the status icons (from Forum Manager > edit forum > click help icon, ?, next to "Prefix for Forum Status Images"):
Code:
- forum_link-48.png
- forum_new-48.png
- forum_new_lock-48.png
- forum_old-48.png
- forum_old_lock-48.png
The green arrow is part of the thread status icon. You would create a plain icon and then one with the green arrow, if you wish to indicate the threads a user has posted in.
And the small ones for the legend that have the number 16 in place of 48.