Hi Kirsty.
here's a mod that does exactly this. Works very nicely and is pretty straightforward. It's a plugin with a couple of template changes, so the php files themselves are left alone.
Once you install it and make the template tweaks, you just go to the forum manager (within the admin control panel) and select each forum that you want to have unique statusicons. Within the setup form you'll now find a switch where you can choose to use custom icons, as well as fields where you enter the names of the images you plan to use for "new" "old" "locked" and "old locked."
The detailed instructions are inside the zip file attachment at the link above. Hope this helps!