I'm not sure what you mean. If you look at the page source, you should see something like this:
<img id="forum_statusicon_37" border="0" alt="" src="images/statusicon/forum_old.gif"/>
That is the path to the image. You can now ftp to the site, go to that directory, and upload a new image using the same name and it should now appear on your forum (you may have to do a Force Refresh of the page to see the image).