I think you are using the wrong terminology... and so I am unsure what you are actually asking.
1- board icon: next to the board name in the board index. This is handled by on.gif (new messages in board), on2.gif (new messages in a child board, but no new messages in the parent) and off.gif (no new messages in the board or any of its children)
/Themes/YOURTHEMEDIRECTORY/images/on.gif
/Themes/YOURTHEMEDIRECTORY/images/on2.gif
/Themes/YOURTHEMEDIRECTORY/images/off.gif
2- new icon: shown to the right of the thread title on the message index page. This is handled by new.gif.
/Themes/YOURTHEMEDIRECTORY/images/english/new.gif