[@robin-w Moderator 5 years, 1 month ago]

you need to wrap the url into an image tag as in

https://www.w3schools.com/tags/tag_img.asp

so (untested so may need fixing) and image sizing but try

add_action('bbp_template_before_replies_loop', 'bbp_show_extra_fields');
function bbp_show_extra_fields() {
  $topic_id = bbp_get_topic_id();
  $value1 = get_post_meta( $topic_id, 'bbp_extra_field1', true);
  $value2 = get_post_meta( $topic_id, 'bbp_extra_field2', true);
  $img1 = '<img src="'.$value1.'">' ;
  echo "Field 1: ".$img1."<br>";
  echo "Field 2: ".$value2."<br>";
}