We have this on our forums, it is as Lynne said quite easy to do.
Essentially we create 2 gifs, prefix_new.gif and prefix_old.gif 30 px x 30 px I believe ( slightly wider for the prefix_new version
Say we have a forum called The Barbie, in manage forums, we click on that forum, and in Style Options add bbq_
Where bbq is the prefix for our images, see below attatchment
These are found here:
http://www.yobromofo.com/forum/image..._forum_new.gif
http://www.yobromofo.com/forum/image..._forum_old.gif
Then just do the same for each forum, its really quite simple.