here is an example of how to do it:
around line #906 of the default.asp file, find the following:
Response.Write """ bgcolor=""" & strForumCellColor & """> </td>" & vbNewline
and change it to look like this: Response.Write """ bgcolor=""" & strForumCellColor & """>" & getCurrentIcon("icon_folder.gif|15|15","","") & "</td>" & vbNewline
you would just need to change the icon_folder.gif to the name of the icon you want to use. And then change the first 15 to the width of the icon you want to use, and the second 15 to the height of the icon you want to use.