This here is something that might be helpful to some people.
Put it in loop-single-reply.php
$dpa_ua = dpa_get_user_last_unlocked( bbp_get_displayed_user_id());
$dpa_unm = get_the_author_meta("user_login", bbp_get_displayed_user_id());
echo "<a href='https://YOURWEBSITE.com/author/".$dpa_unm."/achievements/' title='Achievements'>".get_the_post_thumbnail( $dpa_ua , 'thumbnail')."</a>";