bbPress.org

It is in loop-single-topic.php and loop-single-forum.php.

In the files you will see something similar to this. In this example I am editing the loop-single-topic.php file.

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

Change it to something like this. You see I just added the extra attribute. 'type' => 'avatar'

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

bbp_author_link