[dannykilla]

simply add a prefix to your subforum.

for an example lets say we have a subforum called "manutd"
Now in:
AdminCP > Forum & Moderators > Forum Manager > choose your sub forum in this case manutd

then in the option > Prefix for Forum Status Images
add a prefix called (this is an example)

manutd_

Now create your images, I have two images and called them the following:

manutd_subforum_new.png
manutd_subforum_old.png

Now upload these images to the directory /images/statusicon/

and thats it, if you want to change the file type of the status images simply change the file name in the code (above) to whatever you want. I have now highlighted that part in red!

I have attached some images to help some of you out!