[@robkk Moderator 11 years ago]

This is the CSS causing the issue. You are stretching a 14px image to 40px, that is why it is so blurry.

#bbpress-forums p.bbp-topic-meta img.avatar, 
#bbpress-forums ul.bbp-reply-revision-log img.avatar, 
#bbpress-forums ul.bbp-topic-revision-log img.avatar, 
#bbpress-forums div.bbp-template-notice img.avatar, 
#bbpress-forums .widget_display_topics img.avatar, 
#bbpress-forums .widget_display_replies img.avatar {
    border: none;
    width: 40px;
}

See if the PHP code snippet in this topic can help you resize the 14px avatars better.

Resizing Avatars