You asked, we listened. Today, we released an update to the Lync 2013 Windows client, which gives users the option to eliminate the images of senders and receivers in IM conversations without disabling them in the contact list. We also grouped all of the IM options in the Options dialog to make them easier to find and made a number of other improvements to our media stack to improve call quality wherever you are.
Turn off the images of senders and receivers in the IM conversation window
Read more at source: http://blogs.office.com/2014/10/14/simpler-better-instant-messaging-options-lync-2013/