I re-saved the.js file and created a new 'avatar.png' but still doesn't work. Did you modify line 420?
/* Creation of an avatar image */ if (item.self == false) { if (item.avatar != "") { avatar_img.style.backgroundImage = "url('"+item.avatar+"')"; avatar_img.onclick = MessagingWidgetsRenderer_avatarClicked; } } else { avatar_img.style.backgroundImage = "url('avatar.png')"; avatar_img.onclick = MessagingWidgetsRenderer_avatarClicked; }