bbPress.org

since you have custom bbpress templates in your theme

you would have to look at the loop-single-topic.php file in your bbpress folder and compare it to this one which is the plugins version of the file

https://raw.githubusercontent.com/ntwb/bbPress/master/src/templates/default/bbpress/loop-single-topic.php

basically you need this for the started by author avatar

<?php do_action( 'bbp_theme_before_topic_started_by' ); ?>

<span class="bbp-topic-started-by"><?php printf( __( 'Started by: %1$s', 'bbpress' ), bbp_get_topic_author_link( array( 'size' => '14' ) ) ); ?></span>

<?php do_action( 'bbp_theme_after_topic_started_by' ); ?>

and this for the freshness author avatar

<?php do_action( 'bbp_theme_before_topic_freshness_author' ); ?>

<span class="bbp-topic-freshness-author"><?php bbp_author_link( array( 'post_id' => bbp_get_topic_last_active_id(), 'size' => 14 ) ); ?></span>

<?php do_action( 'bbp_theme_after_topic_freshness_author' ); ?>