Hi Robbie,
I think I know exactly what you're trying to do.
Those images are called 'status icons', you can set a prefix name for each of your category's status icons, by going to your AdminCP, then going to the
'Forum Manager' under
'Forums and Managers'. In the Forum Manager click on one of your categories and scroll down to the setting called
'Prefix for Forum Status Images ', and set something like 'handshake' for one of your categories for example. This means that the images you need to upload into your /images/statusicon folder need to have the title ' handshakeforum_old.gif ' and ' handshake_new.gif'. You can set both these names to the same image file if you want the same image displayed for both 'old' and 'new' status of your category. After you upload those images with the correct filenames the custom images should appear in your category-listings.
Alternatively, you can use this modification which also allows you to use image formats other than .gif as your status icons:
https://vborg.vbsupport.ru/showthread.php?t=102025