[Rein Masamuri]

Ok, I did a quick put together of code here and I havn't tested it so :S
First off goto your templates and goto the template named phpincludes

at the very bottom put:

//if admin
if ($bbuserinfo['userid'] == 6) 
{ 
//show this button
$specialnav = '<a href="url"><img src="{imagesfolder}/adminimage.gif" alt="hover text" border="0"></a>'; 
} 

//like above, the number is equal to the usergroup - this next one is if moderator
if ($bbuserinfo['userid'] == 7) 
{ 
//then show this image
$specialnav = '<a href="url"><img src="{imagesfolder}/modimage.gif" alt="hover text" border="0"></a>'; 
}

Change the needed items to fit your need and put the $specialnav where you want the image
you can also change the == to < or > if you want a certain range of users to have a certain image.
good luck ^_^