It already works like that, only admins and mods see it, to change it from a simple link to your title image, just change it to fit your titleimage, or logo. It would be easier to just add a lnk or button in the header, but if you want to change it around you're more than welcome.
I think this would work:
Use this code in your phpinclude template
Code:
//Administrative/Moderator Login
if ($bbuserinfo[usergroupid]==6) { //Admins
$adminlogin = "<a href=\"admin/index.php\"><img src=\"{ titleimage}\" border=\"0\"></a>";
} elseif (($bbuserinfo[usergroupid]==5) or ($bbuserinfo[usergroupid]==7)) { //Mods
$adminlogin = "<a href=\"mod/index.php\"><img src=\"{ titleimage}\" border=\"0\"></a>";
} else { //Everyone else
$adminlogin = "<img src=\"{ titleimage}\" border=\"0\">";
}
//End Login Code
Then you'd have to change { titleimage} to $adminlogin. (Admin CP -> Templates -> Search -> Search and replace ALL templates. Be careful!) ....Seach for { titleimage} and replace it with $adminlogin.
Remember to remove the space between { and titleimage}