add following code in your theme’s (child theme’s if using) functions.php
add_filter( 'fep_eb_email_legends', function( $legends, $mgs, $user_email ){
$legends['cus_sender_avatar'] = array(
'description' => __('Sender Avatar', 'front-end-pm'),
'where' => array( 'newmessage', 'reply' ), //where this tag will be used
'replace_with' => $mgs ? get_avatar( $mgs->mgs_author, 64 ) : '',
);
$legends['cus_receiver_avatar'] = array(
'description' => __('Receiver Avatar', 'front-end-pm'),
'where' => array( 'newmessage', 'reply' ), //where this tag will be used
'replace_with' => $mgs ? get_avatar( $user_email, 64 ) : '',
);
return $legends;
}, 10, 3);
Then you can use {{cus_sender_avatar}}
and {{cus_receiver_avatar}}