I have a slimmed down version of my forum sponsor mod. You can check it out  here. It's only been tested with mySql though.
It might be a bit more than you want. 
If you have the banners on site, you could try renaming the banners to the forum id number.
 
The banner for forum 16 would be called 16.gif.
Response.Write  "<img src=""banner_folder/" & Forum_ID & ".gif"" height=""50px"" width=""300px"" title=""title_stuff"" />"
If the banners are not on site and you can't rename them, You could maybe do it with a function in inc_func_common.asp.
function GetBanner(fID)
'## fID = Forum ID number
        dim strBanner :strBanner = ""
        select case fID
                case 1
                        strBanner = strBanner & "<img src=""forum_1_banner.gif"" height=""50px"" width=""300px"" title=""title_stuff"" />"
                case 2
                        strBanner = strBanner & "<img src=""forum_2_banner.gif"" height=""50px"" width=""300px"" title=""title_stuff"" />"
                case else
                        strBanner = strBanner
         end select
         GetBanner = strBanner
end function
And the call it in forum or topic.asp 
Response.Write GetBanner(Forum ID variable)
Just some thoughts