bbp_get_forum_thumbnail_src( $forum_id = null,  $size = null,  $type = null )

function bbp_get_forum_thumbnail_src( $forum_id = null, $size = null, $type = null ) {
	if ( $thumbnail_id = get_post_thumbnail_id( $forum_id ) ) {
		return wp_get_attachment_image_url( $thumbnail_id, $size );
	}

	if ( $group_ids = bbp_get_forum_group_ids( $forum_id ) ) {
		$group_id = $group_ids[0];

		$group_avatar_url = bp_core_fetch_avatar([
			'item_id'       => $group_id,
			'object'        => 'group',
			'type'          => $type,
			'html'          => false,
			'force_default' => false,
		]);

		$group_default_avatar = bp_groups_default_avatar( '', [ 'object' => 'group', 'type' => $type ] );

		if ( $group_avatar_url != $group_default_avatar ) {
			return $group_avatar_url;
		}
	}

	return '';
}