The avatar images are of very low resolution because the img-tag asks for 14px but the img.avatar class says 80px in the buddypress.css.
body #bbpress-forums p.bbp-topic-meta .bbp-author-avatar img.avatar {
position: absolute;
left: 0;
top: 0;
border: 0;
width: 80px;
height: 80px;
margin: 12px 16px;
}
<img src="http://gravatar.com/avatar/a2638ea74baa1745d4b5fba0bab6da22?d=wavatar&s=14&r=G" class="avatar user-102-avatar avatar-14 photo" width="14" height="14" alt="Profilbild von Thommy">