Possibly the most easy way is to edit the core file and add such a stuff there.
bb_func_vthread.php - locate the code which looks like
else {
if($cols[1]!=$l_anonymous) $viewReg=$l_anonymous;
else $viewReg='';
$pLink1='';
$pLink2='';
}
there, you may add something to
$viewReg=$l_anonymous; definition. For example,
$viewReg=$l_anonymous.'<br /><img src=""...>';
Another (non-destructive way) would be to install the Avatars add-on for members and then modify
parseUserInfo_user_custom1 function from bb_plugins.php so it also generates the content for Guests.