/**
* Try something like this to get a users role and check if the role is your premium member role, if it is
* then display the badge. Use this inside loop-single-reply.php inside your theme. NOTE: I haven't tested
* this yet.
*/

$user = new WP_User( bbp_get_reply_author_id ( bbp_get_reply_id() ) );
echo $user->roles[0];

if ( $user_role == "donating_member" ) {
    echo "<img='http://example.com/badge.gif' />";
}