It's not possible to show the email instead of the user_name. But you can change the link.
Open includes/functions.php and replace
$user_profile_link = (!empty($url_show_profile)) ? preg_replace("/{user_id}/", $image_row['user_id'], $url_show_profile) : ROOT_PATH."member.php?action=showprofile&".URL_USER_ID."=".$image_row['user_id'];
with
$user_profile_link = (!empty($url_show_profile)) ? preg_replace("/{user_id}/", $image_row['user_id'], $url_show_profile) : ROOT_PATH."member.php?action=mailform&".URL_USER_ID."=".$image_row['user_id'];
Jan