Just enter into the box (without the quotes): "abc_"
Once you have done that, you exit Admin Control Panel because you don't need to change anything for that forum anymore. Then you create your own images, name your image files:
abc_forum_new.gif
abc_forum_old.gif
(If you click the "?" icon located on the same row as the input box, in the AdminCP, it will show you the help message).