You would have to do it in your template code where you use the path to your images.
Code:
<img src="forums/$stylevar[imgdir_statusicon]/thread$thread[statusicon].gif" title="Test" />
Also, if this is a shared template then you can just add a variable in place of "/forums/" and then only populate it if its the custom page as it will be blank for regular pages and populated for custom
Code:
<img src="$forums/$stylevar[imgdir_statusicon]/thread$thread[statusicon].gif" title="Test" />