[Bellardia]

Go to your header template

Find

<img src="$stylevar[titleimage]" border="0" alt="$vboptions[bbtitle]" />

Replace

<img src="$BannerImg" border="0" alt="$vboptions[bbtitle]" />

Create a Plugin
Hook : global_start
Change 1 to whichever forum ID you want, change Images too!

PHP Code:

if($vbulletin->GPC[forumid] == '1')
{
   $BannerImg = 'special.png';
}
else
{
   $BannerImg = 'default.png';
}