I could be wrong, but edit the forum you want to add a picture to in the admin cp and have a look at the "Style Options", specifically the "Prefix for Forum Status Images" section. The help for that option states the following:
Forum Status images are usually shown alongside the forum in a forum listings to indicate the read status.
This prefix must only contain alphanumerical characters, though the following are also allowed - _ / ., a folder name is also acceptable to be placed within the status icon directory.
The following images are currently in use for status icons, you will need to create images with the specified prefix to match.
forum_link.gif
forum_new.gif
forum_new_lock.gif
forum_old.gif
forum_old_lock.gif
I'm going to guess that is how they are doing it.
** edit **
Yes, that is how they are doing it, as I've tried it on my test forum.
Here's what I did.
1) Opened my forum root (the folder that contains all the vBulletin files) and then went into /images/statusicon/ and created a new folder called "test" (You may want to give the folder the name of the forum though)
2) I then created image files named:
forum_link.gif
forum_new.gif
forum_new_lock.gif
forum_old.gif
forum_old_lock.gif
and put them in my "test" folder
3) I then went into: admincp -> Forums & Moderators -> Forum Manager and then selected the forum I wanted to add the images to and edited it. I then went to the Style Options section and in the "Prefix for Forum Status Images" I entered "test/" into the field. This tells vB to look for a folder called test in the statusicon folder. I then saved the changes and went back and checked my forum listing and it was displaying the icons.
One key thing to remember is that there are meant to be different icons to display different status info, for example, new posts in forum, no new posts in the forum, etc. Thus you may need to modify your images to reflect this, because if you simply duplicate the image and rename it for the different status' then users won't be able to tell the difference between the status', but that might not be important to you.
I hope this helps
