This November's update introduced a new feature called "Photos of Sender/Receiver", and I don't see any interface options to turn it off.
http://blogs.technet.com/b/lync/archive/2013/11/12/november-2013-update-for-the-lync-2013-desktop-client.aspx
Maybe I'm wrong, and there is some way to disable it? It totally bloats the IM window.