Those are forum icons dedicated to saying whether it contains new posts or not. They have a unique file name linked to the coding of the forum software. It would require a lot of coding to make it use a different graphic for one section and another for a different section.
No doubt it could probably be done, but it's a hell of a lot of work for not a great deal of benefit really other than a bit of eye-candy. Bigger fish to fry at the moment.